reserve A for set;
reserve X,Y,Z for set,x,x1,x2,y,y1,y2,z,z1,z2 for object;
reserve u for UnOp of A,
  o,o9 for BinOp of A,
  a,b,c,e,e1,e2 for Element of A;

theorem
  for x, y being object, X, Y, Z being set,
   f being Function of [:X,Y:],Z, g being
  Function st Z <> {} & x in X & y in Y holds (g*f).(x,y) = g.(f.(x,y))
    by FUNCT_2:15,ZFMISC_1:87;
