theorem Th72:
  not x in still_not-bound_in p implies (p '&' Ex(x,q)) <=> Ex(x,p
  '&' q) is valid
proof
  assume not x in still_not-bound_in p;
  then
  (p '&' Ex(x,q)) => Ex(x,p '&' q) is valid & Ex(x,p '&' q) => (p '&' Ex(x
  ,q)) is valid by Th71;
  hence thesis by Lm14;
end;
