Explain the role of lexical analyzer in compiler design. What is input buffering? Describe the buffer-pair scheme with sentinels and explain why it is efficient.

Design a lexical analyzer using transition diagram that recognizes:

• Identifiers (letter followed by letters/digits)
• Integer constants
• Operators: +, -, *, /, <, <=, >, >=, ==, =
• Keywords: if, then, else, while

Show the transition diagram and write pseudocode for the nextToken() function.