Rules for executing pure expressions with exec-expr.
Theorem:
(defthm exec-expr-when-pure (implies (and (syntaxp (quotep e)) (expr-purep e) (integerp limit) (>= limit (expr-pure-limit e)) (compustatep compst) (equal eval (exec-expr-pure e compst)) (expr-valuep eval)) (equal (exec-expr e compst fenv limit) (mv eval compst))))