mk_pabspairSyntax.mk_pabs : term * term -> term
Constructs a paired abstraction.
If M is the tuple (v1,..(..)..,vn), and
N is an arbitrary term, then mk_pabs (M,N)
returns the paired abstraction `\(v1,..(..)..,vn).N`.
Fails unless M is an arbitrarily nested pair composed
from variables, with no repetitions of variables.