theorem Th20:
  for TS being transition-system over F holds x1, x2 ==>. y1, y2,
TS implies x1 in TS & y1 in TS & x2 in E^omega & y2 in E^omega & x1 in dom dom
  (the Tran of TS) & y1 in rng (the Tran of TS)
by Th15;
