Visitor (5.11)

The Visitor pattern is one of the most important --- and most difficult patterns to understand and use. Your instructor has modified exercise 5.3.1 (Little Language) to use a visitor. The visitor provided by the instructor implements the printing of the statements.

You are required to modify the instructor's code in two ways. First, design, code and test a new visitor for interpreting "while" programs. Note that the "expr" package remains unchanged. Then, modify the "expr" package to use a visitor and design, code and test a visitor to evaluate expressions of "while" programs.