theorem
  F is commutative implies F[;](x,f) = F[:](f,x)
proof
  assume
A1: F is commutative;
  per cases;
  suppose
    Y = {};
    hence thesis;
  end;
  suppose
A2: Y <> {};
    now
      let y;
      reconsider x1 = f.y as Element of X by A2,FUNCT_2:5;
      thus (F[;](x,f)).y = F.(x,x1) by A2,Th53
        .= F.(f.y,x) by A1;
    end;
    hence thesis by A2,Th49;
  end;
end;
