Goal:
  
  A : Type
  f : A -> bool
  g : A -> bool
  Hfg : f =1 g
  f' : A -> bool
  g' : A -> bool
  Hfg' : f' =1 g'
  i : A
  ============================
   (predU f f' i = predU g g' i)


DEBUG:Going to execute:
<coq-core.plugins.ssreflect::ssrrewrite@0> $1 $2