theorem Th26:
  p just_once_values x implies for k st k in dom p & k <> x..p holds p.k <> x
proof
  assume
A1: p just_once_values x;
  let k;
  assume
A2: k in dom p & k <> x..p & p.k = x;
  p <- x = x..p by A1,Th25;
  hence thesis by A1,A2,Def3;
end;
