The type of a function which
finds an equal term and returns a theorem stating the equality
If the function c is not only of the right type,
but is actually what we call a conversion, then
(c : conv) (t : term) returns a theorem of the form
|- t = t'
If (roughly speaking) t is of the right form to be passed to c,
but c doesn’t find a desired equal term t'
then c t may raise the exception UNCHANGED in preference to returning
|- t = t, as it can be dealt with more efficiently.