Support Restrictions on Expressions

[pulsipher]

See #346.

[pulsipher]

One complicating factor is that when restricting an expression, it may contain many kinds of variables. Consider the expression:
$aff := y(t, x) + w(t) - s(x) + z(x, t) - 42$
If we we then try to restrict this with $aff(2, 5)$, we have no way of knowing which number corresponds to which infinite parameter.