fact: Hypothesis Pr_rel : forall x1 x2 y1 y2, Pr x1 x2 -> Pr y1 y2 -> Pr (op1 x1 y1) (op2 x2 y2).
imports: ssreflect ssrbool ssrfun eqtype ssrnat seq div choice fintype finfun paths Setoid Morphisms OperationClasses bigops