fact: Definition mobile_alt R := forall E F, inc E R -> inc F R -> E <> F -> forall x y, inc x (E \cap F) -> inc y (E -s F) -> exists G, [/\ inc G R, sub G (E \cup F), inc y G & ~ (inc x G)].
imports: mathcomp(ssreflect ssrfun ssrbool eqtype ssrnat)