PABS : (term -> thm -> thm)
         A |- t1 = t2
   ------------------------  ABS "p"            [Where p is not free in A]
    A |- (\p.t1) = (\p.t2)
EXAMPLE
- PABS (Term `(x:'a,y:'b)`) (REFL (Term `(x:'a,y:'b)`)); > val it = |- (\(x,y). (x,y)) = (\(x,y). (x,y)) : thm