theorem Th26:
  for f being Function st x in dom <:f,X,Y:> holds <:f,X,Y:>.x = f .x
proof
  let f be Function;
  assume
A1: x in dom <:f,X,Y:>;
  then x in dom f & f.x in Y by Th24;
  hence thesis by A1,Th25;
end;
