Given a program in the language (as a string or AST) with a free variable named <var>x</var> (or another name if that is not valid syntax), evaluate it with <var>x</var> bound to a provided value, then evaluate it again with <var>x</var> bound to another provided value, then subtract the result of the first from the second and return or print it.

Do so in a way which:
* does not involve string manipulation of the input source code
* is plausibly extensible to a runtime-chosen set of bindings rather than just <var>x</var>
* does not make <var>x</var> a ''global'' variable
or note that these are impossible.

===See also===
* For more general examples and language-specific details, see [[Eval]].
* [[Dynamic variable names]] is a similar task.