es12


   b:t1->t2,c:t1, y:t2 |- c:t1      b:t1->t2,c:t1 |- b:t1->t2   b: t1->t2, c: t1 |- c: t1
  ------------------------------    -------------------------------------------------------
   b:t1->t2, c:t1 |- \y.c:t2->t1                b: t1->t2, c: t1 |- (bc): t2
  ---------------------------------------------------------------------------
                 b:t1->t2, c:t1 |- (\y.c)(bc): t1
             -------------------------------------------
               c:t1 |- \b.((\y.c)(bc)): (t1->t2)->t1


