Try making command constructors more readable.
Is it worthwhile to format this stuff?

I can't decide. Let's try it out and see how it goes.
Add missing definitions for bind.
Remove unused code.

On second thought, I'm never going to use these definitions
for bind, so I might as well get rid of them.
Handle yield statements in evaluator.
Evaluate alias expressions.
Move more logic from Interpreter into Evaluator (work in progress).
Fiddle with Evaluator module.
Variables may be associated with a heap.
Implement "new" expression in Evaluator.
Make Value an instance of Show.
Evaluate LoadMember expressions.
Implement method calls.
Try reformatting.
Implement StoreMember statement in Evaluator.
Implement StoreElement statement in Evaluator.
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