Goal:
    f : Edge.t -> Edge.t  es : EdgeSet.t  e : EdgeSet.elt
  ============================
   (EdgeSet.In e (edge_map f es) ->
    exists e0 : Edge.t, e = f e0 /\ EdgeSet.In e0 es)


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