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CPW Pt. 8: Representing Assignment Expressions				[TOC]
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---- VIDEO ------------------------------
https://www.youtube.com/embed/h8FP63W1PRY
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Quiz 21
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Extend the interface and implementation for expression nodes so that assignments
and identifiers are supported:

- Provide a constructor _newIdentifierExpr()_ for creating a dynamically
  allocated expression node for an identifier specified as parameter. The
  identifier is a unique string.
- Provide a method _isLValueExpr()_ that returns true if an expression node
  represents an identifier.
- Provide a enum constant _EK_ASSIGN_ for an expression kind that represents a
  binary node for an assignment.
- Change the implementations of all methods so that the new kinds of expression
  nodes are supported. In the implementation of _evalExp_ you can assume that
  for the identifier a symbol exists. If this is not the case your code should
  trigger an assertion.

Write a test program ~xtest_expr.c~.

---- CODE (type=sh) ------------------------------------------------------------------------------------------
submit hpc quiz21 expr.h expr.c sym.h sym.c ustr.h ustr.c xtest_expr.c lexer.h lexer.c tokenkind.h tokenkind.c
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