theorem Th28:
  for TS being transition-system over F st not <%>E in rng dom (
  the Tran of TS) holds not x, z ==>. y, z, TS
proof
  let TS be transition-system over F such that
A1: not <%>E in rng dom (the Tran of TS);
  assume x, z ==>. y, z, TS;
  then consider v, w such that
A2: v = z and
A3: x, w -->. y, TS and
A4: z = w^v;
  [[x, w], y] in the Tran of TS by A3;
  then
A5: [x, w] in dom (the Tran of TS) by XTUPLE_0:def 12;
  w = <%>E by A2,A4,FLANG_2:4;
  hence contradiction by A1,A5,XTUPLE_0:def 13;
end;
