ALPHA_CONV
Drule.ALPHA_CONV : term -> conv
Renames the bound variable of a lambda-abstraction.
If x is a variable of type ty and M is an abstraction (with bound
variable y of type ty and body t), then ALPHA_CONV x M returns
the theorem:
|- (\y.t) = (\x'. t[x'/y])
where the variable x':ty is a primed variant of x chosen so as not
to be free in \y.t.
Failure
ALPHA_CONV x tm fails if x is not a variable, if tm is not an
abstraction, or if x is a variable v and tm is a lambda
abstraction \y.t but the types of v and y differ.