default "absurd"
cic:/Coq/Init/Logic/absurd.con.
-interpretation "Coq's leibnitz's equality" 'eq x y = (cic:/Coq/Init/Logic/eq.ind#xpointer(1/1) _ x y).
+interpretation "Coq's leibnitz's equality" 'eq x y = (cic:/Coq/Init/Logic/eq.ind#xpointer(1/1) ? x y).
theorem f_equal1 : \forall A,B:Type.\forall f:A\to B.\forall x,y:A.
x = y \to (f y) = (f x).