Lazy expressions

A primary expression (or any other expression in parentheses) may be modified to be lazily evaluated. Instead of evaluating the expresssion on the spot, a thunk is created and returned.

This thunk reflects the return value of an expression, so that upon first use, its value is evaluated, stored, and returned.

Thunks are implemented as an anonymous nested function enclosed in the thunk ref.

  lazy-expr := "^" primary 

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