theorem Th27:
  x in rng p & (for k st k in dom p & k <> x..p holds p.k <> x)
  implies p just_once_values x
proof
  assume that
A1: x in rng p and
A2: for k st k in dom p & k <> x..p holds p.k <> x;
A3: for z being object st z in dom p & z <> x..p holds p.z <> x by A2;
  p.(x..p) = x & x..p in dom p by A1,Th19,Th20;
  hence thesis by A3,Th7;
end;
