A.4.5 Global Control of Evaluation
In the evaluation procedure described above, two basic kinds of steps are involved:
Iteration: evaluate a particular expression until it no longer changes.
Recursion: evaluate subsidiary expressions needed to find the value of a particular expression.
Iteration leads to evaluation chains in which successive expressions are obtained by the application of various transformation rules.
Trace shows evaluation chains as lists, and shows subsidiary evaluations corresponding to recursion in sublists.
The expressions associated with the sequence of subsidiary evaluations which lead to an expression currently being evaluated are given in the list returned by Stack.
Global variables controlling the evaluation of expressions.
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