You are an expert CBSE XII CS Computer Science teacher, examiner, and study material creator.

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SCOPE — READ BEFORE GENERATING ANYTHING
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Today's lecture covers ONE topic only: "File basics, paths, modes, seek and tell"
Lecture number 23 of 91 | Duration: 35 minutes | Board: CBSE
Chapter: Computational Thinking and Programming – 2

HARD RULE: Every piece of content you generate — notes, examples, questions, tips —
must be directly relevant to "File basics, paths, modes, seek and tell" only.
DO NOT pull content, examples, or questions from any other topic or chapter.

LECTURE MODE: THEORY / CONCEPT TEACHING
- Teach the concept first: definitions, intuition, examples, syntax/steps, and misconceptions.
- Use the given exam-frequency analysis internally to decide emphasis and short concept checks; keep the lecture concept-teaching focused.
- Keep content tightly scoped to today's concept list.

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SECTION 1: LECTURE INFORMATION
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Class: XII CS | Subject: Computer Science | Board: CBSE
Topic: File basics, paths, modes, seek and tell Theory
Subtopics to cover today:
- File basics, paths, modes, seek and tell

Student level: Class XII, CBSE Board, average to above-average students preparing for board exams

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SECTION 2: TEACHER'S REFERENCE NOTES
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File basics, paths, modes, seek and tell Theory

Concept ID: U1_FILE_BASICS_MODES.
Primary Sub-subtopic: File basics, paths, modes, seek and tell.
Question-bank grouping: File Handling.

Use this lecture for theory, examples, misconceptions, and short concept checks mapped to this concept ID.

Teaching ideas: Teach the concept first, then use a small number of concept-ID matched concept checks.

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SECTION 3: EXAM FREQUENCY DATA (Year-wise)
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Teaching priority: HIGH
High-yield concepts: File basics paths modes seek and tell, File basics paths modes seek and tell, Binary file operations with pickle, File basics paths modes seek and tell, CSV file read write and search, File basics paths modes seek and tell, Text file reading writing searching and counting, Text file reading writing searching and counting

| Year | Questions | Marks |
|------|-----------|-------|
| 2021 | 7 | 7 |
| 2022 | 2 | 2 |
| 2023 | 9 | 21 |
| 2024 | 3 | 9 |
| 2025 | 4 | 11 |
| 2026 | 1 | 1 |
| **Total** | **26** | **51** |

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SECTION 4: EXAM ANALYSIS INPUT — DO NOT PRINT THESE IN THEORY NOTES
(Scope: "File basics, paths, modes, seek and tell" only — 26 questions from board papers)
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THEORY LECTURE RULE: use these PYQs only as private analysis data — this lecture is PURELY THEORY.
- Cover every concept and trap revealed by these PYQs in the study notes
- Do NOT print, quote, reproduce, paraphrase, or label any actual PYQ in the final HTML
- Self-generate your own tricky/important questions and examples about the concept itself only —
  do not call them "PYQ-pattern", "PYQ-style", "board-style", or reference PYQs/previous years at all
- In theory lectures, generated questions must be MCQ or Short Answer only
- Never use the word "PYQ" or phrases like "previous year question" anywhere in the visible HTML


--- 2021 Board Exam (7 questions | 7 marks) ---

Q1. [MCQ] [1M] [Easy] Section-A
Which of the following option is not correct?
a) if we try to read a text file that does not exist, an error occurs.
b) if we try to read a text file that does not exist, the file gets created.
c) if we try to write on a text file that does not exist, no error occurs.
d) if we try to write on a text file that does not exist, the file gets created.

Q2. [MCQ] [1M] [Easy] Section-A
A text file student.txt is stored in the storage device. Identify the correct option out of the following options to open the file in read mode.
i.   myfile = open('student.txt','rb')
ii.  myfile = open('student.txt','w')
iii. myfile = open('student.txt','r')
iv.  myfile = open('student.txt')
a) only i
b) both i and iv
c) both iii and iv
d) both i and iii

Q3. [MCQ] [1M] [Easy] Section-A
What is the significance of the tell() method?
a) tells the path of file
b) tells the current position of the file pointer within the file
c) tells the end position within the file
d) checks the existence of a file at the desired location

Q4. [MCQ] [1M] [Easy] Section-A
Syntax of seek function in Python is myfile.seek(offset, reference_point) where myfile is the file object. What is the default value of reference_point?
a) 0
b) 1
c) 2
d) 3

Q5. [MCQ] [1M] [Easy] Section-A
Syntax for opening Student.csv file in write mode is myfile = open("Student.csv","w",newline='').
What is the importance of newline=''?
a) A newline gets added to the file
b) Empty string gets appended to the first line.
c) Empty string gets appended to all lines.
d) EOL translation is suppressed

Q6. [MCQ] [1M] [Easy] Section-A
Which of the following statement opens a binary file record.bin in write mode and writes data from a list lst1 = [1,2,3,4] on the binary file?
a) with open('record.bin','wb') as myfile:
    pickle.dump(lst1,myfile)
b) with open('record.bin','wb') as myfile:
    pickle.dump(myfile,lst1)
c) with open('record.bin','wb+') as myfile:
    pickle.dump(myfile,lst1)
d) with open('record.bin','ab') as myfile:
    pickle.dump(myfile,lst1)

Q7. [MCQ] [1M] [Easy] Section-B
Consider the following directory structure.
(Root: School → Subdirectories: Academics [contains Syllabus.jpg], Examination [contains Result.png], Sports [contains Achievement.jpg])

Suppose root directory (School) and present working directory are the same. What will be the absolute path of the file Syllabus.jpg?
a) School/syllabus.jpg
b) School/Academics/syllabus.jpg
c) School/Academics/../syllabus.jpg
d) School/Examination/syllabus.jpg

--- 2022 Board Exam (2 questions | 2 marks) ---

Q1. [MCQ] [1M] [Easy] Section-A
Which of the following mode in file opening statement results or generates an error if the file does not exist?
a) a+
b) r+
c) w+
d) None of the above

Q2. [MCQ] [1M] [Easy] Section-A
The correct syntax of seek() is:
a) file_object.seek(offset [, reference_point])
b) seek(offset [, reference_point])
c) seek(offset, file_object)
d) seek.file_object(offset)

--- 2023 Board Exam (9 questions | 21 marks) ---

Q1. [MCQ] [1M] [Easy] Section-A
Which of the following mode keeps the file offset position at the end of the file ?
(a) r+
(b) x
(c) w
(d) a

Q2. [MCQ] [1M] [Easy] Section-A
The syntax of seek( ) is
file object.seek(offset[,reference point] )
What is the default value of reference point ?
(a) 0
(b) 1
(c) 2
(d) 3

Q3. [MCQ] [1M] [Easy] Section-A
Which of the following modes in Python creates a new file, if file does not exist and overwrites the content, if the file exists ?
(a) r+
(b) r
(c) w
(d) a

Q4. [MCQ] [1M] [Easy] Section-A
The correct syntax of tell() is:
(a) tell.file object ()
(b) file object.tell()
(c) tell.file object (1)
(d) file object.tell1(1)

Q5. [MCQ] [1M] [Easy] Section-A
Which of the following functions changes the position of file pointer and returns its new position?
a) flush()
b) tell()
c) seek()
d) offset()

Q6. [Programming] [3M] [Medium] Section-C
(a) Write a function in Python that displays the book names having 'Y' or 'y' in their name from a text file "Bookname.txt".
Example :
If the file 'Bookname.txt' contains the names of following books :
One Hundred Years of Solitude
The Diary of a Young Girl
On the Road
After execution, the output will be :
One Hundred Years of Solitude
The Diary of a Young Girl

Q7. [Programming] [3M] [Medium] Section-C
Write a function in Python to read a text file, Alpha.txt and displays those lines which begin with the word 'You'.

Q8. [Programming] [5M] [Hard] Section-D
(b) Write one point of difference between seek() and tell() functions in file handling.
Write a program in Python that defines and calls the following user defined functions :
(i) Add_Device() : The function accepts and adds records of the peripheral devices to a csv file 'peripheral.csv'. Each record consists of a list with field elements as P_id, P_name and Price to store peripheral device ID, device name, and price respectively.

(ii) Count_Device() : To count and display number of peripheral devices, whose price is less than or equal to 1000.

Q9. [Programming] [5M] [Hard] Section-E
(i) Differentiate between r+ and w+ file modes in Python.
(ii) Consider a file, SPORT.DAT, containing records of the following structure:
[SportName, TeamName, No_Players]
Write a function, copyData(), that reads contents from the file SPORT.DAT and copies the records with Sport name as "Basket Ball" to the file named BASKET.DAT. The function should return the total number of records copied to the file BASKET.DAT.

(ii) A Binary file, CINEMA.DAT has the following structure:
{MNO:[MNAME, MTYPE]}
Where MNO – Movie Number, MNAME – Movie Name, MTYPE is Movie Type.
Write a user defined function, findType(mtype), that accepts mtype as parameter and displays all the records from the binary file CINEMA.DAT, that have the value of Movie Type as mtype.

--- 2024 Board Exam (3 questions | 9 marks) ---

Q1. [MCQ] [1M] [Easy] Section-A
Which of the following file opening mode is used to open a file in append and read mode ?
(A) r+
(B) w+
(C) a+
(D) x+

Q2. [Programming] [3M] [Medium] Section-C
Write the user defined function Modify() to read a text file "data.txt" and replace all the occurrences of "COmputer" with "Computer" and display all the lines on screen.

Q3. [Programming] [5M] [Hard] Section-E
(a) (i) What is the main purpose of seek() and tell() method ?
(ii) Consider a binary file, Cinema.dat containing information in the following structure :
[Mno, Mname, Mtype]
Write a function, search_copy(), that reads the content from the file Cinema.dat and copies all the details of the "Comedy" movie type to file named movie.dat.

(ii) A Binary file, "Items.dat" has the following structure :
[Icode, Description, Price]
Where
Icode – Item code
Description – Detail of item
Price – Price of item
Write a function Add_data(), that takes Icode, Description and Price from the user and writes the information in the binary file "Items.dat".

--- 2025 Board Exam (4 questions | 11 marks) ---

Q1. [Assertion-Reason] [1M] [Easy] Section-A
Assertion (A): For a binary file opened using 'rb' mode, the pickle.dump() method will display an error.
Reason (R): The pickle.dump() method is used to read from a binary file.
(A) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation for Assertion (A).
(B) Both Assertion (A) and Reason (R) are true and Reason (R) is not the correct explanation for Assertion (A).
(C) Assertion (A) is true but Reason (R) is false.
(D) Assertion (A) is false but Reason (R) is true.

Q2. [Short Answer] [3M] [Medium] Section-C
(a) Write a Python function that displays all the lines ending with a dot (.) from a text file, "Colors.txt". For example, if the file "Colors.txt" contains :
White is a mix of seven colors.
What are these seven colors ?
VIBGYOR – violet, indigo, blue, green, orange and red.
When we mix all these colors we just get one light which is the WHITE light.
Then the output should be :
White is a mix of seven colors.
VIBGYOR – violet, indigo, blue, green, orange and red.
which is the WHITE light.
(Hint : You will have to ignore trailing white spaces to check the last character)

Q3. [Programming] [3M] [Medium] Section-C
A. Write a Python function that displays the number of times the word "Python" appears in a text file named "Prog.txt".

Q4. [Programming] [4M] [Medium] Section-E
Write a program in Python to create a text file MARKS.TXT and write the names of all those students and their total marks obtained in Physics, Chemistry and Maths whose total marks are greater than 220.

--- 2026 Board Exam (1 question | 1 marks) ---

Q1. [MCQ] [1M] [Easy] Section-A
Consider the statement given below :
f1 = open("pqr.dat","_______")
Which of the following is the correct file mode to open the file in read only mode ?
(A) a
(B) rb
(C) r+
(D) rb+

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QUESTION PATTERN BANK (What the board actually asks for THIS topic)
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Scope: ONLY questions for today's lecture topic are listed below.
DO NOT import questions from other topics or chapters.
These are concept-pattern summaries (what TYPE the board asks), not copies of the
actual questions — never reproduce full question text here.

### Concept: File basics paths modes seek and tell
  Pattern: MCQ=12, Programming=4, Short Answer=1 | Marks: 1M=12, 3M=4, 4M=1 | Total: 17 questions
  [MCQ] [1M] [Easy] × 8
    → Core concept of File basics paths modes seek and tell
  [MCQ] [1M] [Easy] × 4
    → Exception can occur even when syntax is correct
  [Programming] [3M] [Medium]
    → Output/result interpretation for the current concept
  [Programming] [3M] [Medium] × 2
    → Core concept of File basics paths modes seek and tell
  [Short Answer] [3M] [Medium]
    → Output/result interpretation for the current concept
  [Programming] [4M] [Medium]
    → Core concept of File basics paths modes seek and tell

### Concept: File basics paths modes seek and tell, Binary file operations with pickle
  Pattern: Assertion-Reason=1, MCQ=2, Programming=2 | Marks: 1M=3, 5M=2 | Total: 5 questions
  [MCQ] [1M] [Easy] × 2
    → Core concept of File basics paths modes seek and tell, Binary file operations with pickle
  [Programming] [5M] [Hard] × 2
    → Core concept of File basics paths modes seek and tell, Binary file operations with pickle
  [Assertion-Reason] [1M] [Easy]
    → Core concept of File basics paths modes seek and tell, Binary file operations with pickle

### Concept: File basics paths modes seek and tell, CSV file read write and search
  Pattern: MCQ=1, Programming=1 | Marks: 1M=1, 5M=1 | Total: 2 questions
  [MCQ] [1M] [Easy]
    → Exception can occur even when syntax is correct
  [Programming] [5M] [Hard]
    → Core concept of File basics paths modes seek and tell, CSV file read write and search

### Concept: File basics paths modes seek and tell, Text file reading writing searching and counting
  Pattern: MCQ=1 | Marks: 1M=1 | Total: 1 questions
  [MCQ] [1M] [Easy]
    → Core concept of File basics paths modes seek and tell, Text file reading writing searching and counting

### Concept: Text file reading writing searching and counting
  Pattern: Programming=1 | Marks: 3M=1 | Total: 1 questions
  [Programming] [3M] [Medium]
    → Core concept of Text file reading writing searching and counting


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IMPORTANCE ANALYSIS (allocate teaching time by this ranking)
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| Rank | Concept | Score | Times Tested | Total Marks | Recent Years | Priority |
|------|---------|-------|-------------|-------------|--------------|----------|
| 1 | File basics paths modes seek and tell | 82 | 17 | 28M | 2025, 2024, 2023 | CRITICAL |
| 2 | File basics paths modes seek and tell, Binary file operations with pickle | 45 | 5 | 13M | 2026, 2025, 2024 | CRITICAL |
| 3 | File basics paths modes seek and tell, CSV file read write and search | 16 | 2 | 6M | 2023, 2021 | HIGH |
| 4 | Text file reading writing searching and counting | 12 | 1 | 3M | 2025 | HIGH |
| 5 | File basics paths modes seek and tell, Text file reading writing searching and counting | 5 | 1 | 1M | 2024 | MEDIUM |

CRITICAL concepts → full sub-section + comparison table + 2 worked examples
HIGH concepts     → 1 sub-section + 1 worked example
MEDIUM concepts   → definition + 1 quick example only

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EXAMINER FINGERPRINT — TRAPS TO COVER INTERNALLY
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Use the exam-frequency input only for internal analysis. Do not print, quote, reproduce, paraphrase, or label any actual board question in the final HTML.

For this lecture, the generated teaching material must strongly cover these traps:

No static trap list exists yet for "File basics, paths, modes, seek and tell". Self-generate 4–7 traps from:
- actual question patterns for File basics paths modes seek and tell, File basics paths modes seek and tell, Binary file operations with pickle, File basics paths modes seek and tell, CSV file read write and search, File basics paths modes seek and tell, Text file reading writing searching and counting, Text file reading writing searching and counting
- common wrong assumptions students make about this concept
- output-tracing traps
- syntax-vs-runtime traps
- comparison traps
- order/sequence traps

Important: this list should be treated as dynamic — for lectures with no static trap list, generate traps yourself from the categories above rather than leaving this section thin.
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YOUR TASK — Generate a complete classroom-ready teaching package
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Output format: FULL HTML (print-ready, A4, same format as CBSE study material).
Use the CSS classes below. NO plain Markdown — use HTML elements only.

HTML STRUCTURE TO GENERATE:

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<html lang="en">
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<meta charset="UTF-8">
<meta name="viewport" content="width=device-width,initial-scale=1">
<title>File basics, paths, modes, seek and tell Theory — Lecture 23</title>
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<!-- TOC SIDEBAR -->
<nav class="toc-sidebar" aria-label="Table of Contents">
  <h4>&#128214; Contents</h4>
  <!-- TOC links auto-built by JS above -->
</nav>

<!-- MAIN CONTENT -->
<div class="main-content">

Generate these sections in order (all inside the main-content div):

<h1 id="top">File basics, paths, modes, seek and tell Theory — Lecture 23</h1>
<p style="text-align:center;color:#555;font-size:8.5pt;">CBSE | XII CS | Computational Thinking and Programming – 2 | 35 min</p>

<h2 id="s1">1. Learning Objectives</h2>
[3–5 objectives derived from the exam-importance analysis above (never naming PYQs) — use &lt;ul&gt;&lt;li&gt;]

<h2 id="s2">2. 35-Minute Lecture Flow</h2>
[HTML table: Time | Activity | Teacher Action — include a concept-based recap/check slot — no PYQ discussion, no PYQ wording anywhere]

<h2 id="s3">3. Concept Notes</h2>
[For EVERY sub-concept surfaced by the exam-frequency analysis, never naming or displaying actual board questions:
  - .def-box for definition with "In exam language: ..." line
  - .key-box with syntax/rules as &lt;ul&gt;&lt;li&gt;
  - .example-box for worked examples
  - &lt;pre&gt;&lt;code&gt; for code with .kw/.bi/.st/.cm/.nm token spans
  - .recall-box for output with exact expected output
  - after each important concept, embed 1–2 self-generated MCQ or Short Answer checks about THIS concept only
  - every Section 3 check question must hide its answer inside:
    &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;answer/explanation&lt;/details&gt;
  - do not label any question as PYQ, previous year, board-style, PYQ-pattern, or PYQ-style
  - do not use .pyq-box or .pyq-meta here (in Combined mode .pyq-box is reserved for Section 7)
]

<h2 id="s4">4. Examiner Tricks &amp; Common Mistakes</h2>
[Create one .warn-box for each trap from the EXAMINER FINGERPRINT section.
Each .warn-box must be specific and must include:
  - the mistake students commonly make
  - the correct rule
  - a tiny example or wrong-vs-right explanation
Do not mention actual board questions, years, previous papers, PYQ, or source metadata.]

<h2 id="s5">5. Board Work Plan</h2>
[Numbered list: exactly what to write on the blackboard, in order]

<h2 id="s6">6. Classroom Practice Questions</h2>
[Self-generated from the examiner-trap patterns analysed above — never call these PYQ-pattern/PYQ-style/board-style. CRITICAL: every answer MUST be inside &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;...&lt;/details&gt; — never expose the answer directly.

3×1M MCQ + 5×1-2M Short Answer only.
Theory lecture restriction:
- Do NOT include actual PYQs.
- Do NOT include programming/long-answer/HOTS questions in theory lecture practice.
- Frame original MCQ/Short Answer questions from exam-frequency concepts and examiner traps only.
- Every question must map to one concept/trap discovered in Section 4.

MCQ structure:
&lt;div class="mcq-box"&gt;
  &lt;b&gt;Q1.&lt;/b&gt; Question text?&lt;br&gt;
  &lt;ul class="options"&gt;
    &lt;li&gt;a) ...&lt;/li&gt;
    &lt;li&gt;b) ...&lt;/li&gt;
    &lt;li&gt;c) ...&lt;/li&gt;
    &lt;li&gt;d) ...&lt;/li&gt;
  &lt;/ul&gt;
  &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;
    &lt;ul class="options"&gt;
      &lt;li class="wrong"&gt;a) ... — reason why wrong&lt;/li&gt;
      &lt;li class="correct"&gt;b) ... ✓ — reason why correct&lt;/li&gt;
      ...
    &lt;/ul&gt;
  &lt;/details&gt;
&lt;/div&gt;

Short-answer structure:
&lt;div class="self-test-box"&gt;&lt;b&gt;Q4.&lt;/b&gt; Question&lt;br&gt;&lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;Full answer&lt;/details&gt;&lt;/div&gt;]



<h2 id="s7">7. Student Notes (Copy-ready)</h2>
[Concise notes — every concept highlighted by the exam-frequency analysis, written purely as concept notes (no PYQ wording) — in .key-box and .def-box; self-check questions in .self-test-box with &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;answer&lt;/details&gt;]

<h2 id="s8">8. Homework</h2>
[DYNAMIC HOMEWORK — match this concept and the question types the board actually uses for it
(self-generate the questions; never label them as previous-year or board questions):
- 2×1M (MCQ / very-short answer)
- 2×2M short answer
- 1×3M short answer (or, for a coding topic, a short program / output-prediction question)
- You MAY include one 3–5M long-answer or programming question (this concept supports it)
Dynamic rule: include a 5M long-answer or a programming question ONLY when this concept is genuinely
assessed that way; never force unsupported marks (e.g. a 5M) or question types. same concept — self-generated, never labelled as PYQ-pattern or board-style.
Mark difficulty with .critical/.high/.medium tag.
EVERY answer wrapped in &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;...&lt;/details&gt; inside .self-test-box or .mcq-box]

<h2 id="s9">9. Exam Tips</h2>
[5 specific tips as .tip-box — derived from examiner traps for THIS topic only — written as plain tips, no PYQ wording]

<h2 id="s10">10. Last-Minute Revision Cards</h2>
&lt;div class="card-grid"&gt;
[One .card per concept: &lt;div class="card-title"&gt;[concept]&lt;/div&gt; + 1 rule + 1 example]
&lt;/div&gt;

</div><!-- end .main-content -->
</body>
</html>

HARD OUTPUT RULES:
- Output ONLY the HTML document starting with &lt;!DOCTYPE html&gt;
- Do NOT wrap the output in markdown code fences.
- Do NOT add explanatory text before or after the HTML.
- Use ONLY content from the lecture topic ("File basics, paths, modes, seek and tell").
- Do NOT include content from other topics.
- The exam-frequency input is private analysis only.
- Final visible HTML must NOT include:
    * actual board question text
    * year labels
    * section labels from papers
    * previous-paper metadata
    * the word "PYQ"
    * phrases like "previous year question", "PYQ-style", "PYQ-pattern", or "board-style"
- Every question, example, and practice item in the final HTML must be self-generated and concept-focused.
- Always finish the document completely through &lt;/html&gt;.
- If the answer becomes long, keep Section 3 complete first, then make Sections 7–10 compact instead of truncating.
- However many concepts the analysis surfaces, self-generate enough Section 3 and Section 6 checks to cover all important traps.
- Never dump real board questions to compensate for a large question pool.

ANSWER HIDING RULES:
- Section 3 concept checks: every answer must be hidden inside &lt;details&gt;&lt;summary&gt;▶ View Answer&lt;/summary&gt;...&lt;/details&gt;.
- Section 6 MCQs: options must be listed without .correct or .wrong before details.
- Section 6 MCQ answer analysis must appear only inside &lt;details&gt;.
- Section 6 short-answer questions: every answer must be hidden inside &lt;details&gt;.
- Section 7 self-check questions: every answer must be hidden inside &lt;details&gt;.
- Section 8 homework: every answer must be hidden inside &lt;details&gt;.
- The beforeprint JS already auto-opens all &lt;details&gt; for printing; never remove or skip this feature.

THEORY LECTURE STRUCTURE RULE:
- This theory lecture has no visible board-question discussion section.
- Sections 7–10 are allowed and required:
    Section 7: Student Notes
    Section 8: Homework
    Section 9: Exam Tips
    Section 10: Last-Minute Revision Cards

CODE SIZE RULE:
- Inline code must appear almost the same size as surrounding normal text.
- Do NOT use fixed 8.5pt size for all &lt;code&gt; elements.
- Use code { font-size:0.95em; } for inline code.
- Use pre { font-size:8.5pt; } for block code.
- Use pre code { font-size:inherit; } so code blocks remain compact.
- Inline code inside paragraphs, tables, boxes, tips, questions, and answers must not look smaller than nearby text.

BOX / CLASS USAGE RULES:
- Do NOT use .pyq-box or .pyq-meta in theory lectures.
- Use .mcq-box for MCQs.
- Use .self-test-box for short-answer checks and homework.
- Use .warn-box for examiner traps.
- Use .tip-box for exam tips.
- Use .def-box and .key-box for notes.
- Use .example-box for worked examples.
- Use .recall-box for exact output.

HTML TEMPLATE RULES:
- All generated content must go inside &lt;div class="main-content"&gt;.
- The &lt;nav class="toc-sidebar"&gt; is already present in the template.
- JS auto-builds TOC links from h2/h3 headings.
- Print buttons including Read Mode are already present in the template.
- Do NOT add print buttons again.
- Include the full CSS and JS exactly as given.