GEN_ALPHA_CONV : (term -> conv)
If tm is an abstraction "\y.t" or an application of a binder to
an abstraction "B y.t", where the bound variable y has type ":ty",
and if "x" is a variable also of type :ty, then GEN_ALPHA_CONV "x" tm
returns one of the theorems:
|- (\y.t) = (\x'. t[x'/y])
|- (B y.t) = (!x'. t[x'/y])
depending on whether the input term is "\y.t" or "B y.t"
respectively. The variable x':ty in the resulting theorem is a primed
variant of x chosen so as not to be free in the term provided as the second
argument to GEN_ALPHA_CONV.