====== Metaterms ======

A *metaterm* is a literal that represents a term.  You can think of it as a fancy sort of string.  Metaterms are created with the at symbol (''@'', U+0040).

Consider a case where we need to know if a term is a variable.  We might have an operator like ''is_variable'' that takes a single argument and evaluates to ''true'' when the argument is a variable, and to ''false'' when it is not.  We might use that in a lambda, but if we do, we have to *defer evaluation* until we are ready.

    is_variable($x)

Here this will *immediately* be rewritten to ''true'' because