theorem Th28:
  for f being Function st Z c= X holds <:f,Z,Y:> c= <:f,X,Y:>
proof
  let f be Function such that
A1: Z c= X;
A2: dom <:f,Z,Y:> c= dom <:f,X,Y:>
  proof
    let x be object;
    assume
A3: x in dom <:f,Z,Y:>;
    then
A4: f.x in Y by Th24;
    x in Z & x in dom f by A3,Th24;
    hence thesis by A1,A4,Th24;
  end;
  now
    let x be object;
    assume
A5: x in dom <:f,Z,Y:>;
    then <:f,Z,Y:>.x = f.x by Th26;
    hence <:f,Z,Y:>.x = <:f,X,Y:>.x by A2,A5,Th26;
  end;
  hence thesis by A2,GRFUNC_1:2;
end;
