
theorem
  for A,B be category, F,F1,F2,F3 be covariant Functor of A,B for t be
  natural_transformation of F,F1, t1 be natural_transformation of F1,F2 holds F
  is_naturally_transformable_to F1 & F1 is_naturally_transformable_to F2 & F2
is_naturally_transformable_to F3 implies for t3 being natural_transformation of
  F2,F3 holds t3`*`t1`*`t = t3`*`(t1`*`t)
proof
  let A,B be category, F,F1,F2,F3 be covariant Functor of A,B;
  let t be natural_transformation of F,F1, t1 be natural_transformation of F1,
  F2;
  assume that
A1: F is_naturally_transformable_to F1 and
A2: F1 is_naturally_transformable_to F2 and
A3: F2 is_naturally_transformable_to F3;
A4: F is_naturally_transformable_to F2 by A1,A2,Th8;
  let t3 be natural_transformation of F2,F3;
A5: F2 is_transformable_to F3 by A3;
A6: F is_transformable_to F1 & F1 is_transformable_to F2 by A1,A2;
  F1 is_naturally_transformable_to F3 by A2,A3,Th8;
  hence t3`*`t1`*`t = t3`*`t1`*`(t qua transformation of F,F1) by A1,Def8
    .= (t3 qua transformation of F2,F3)`*`t1`*`t by A2,A3,Def8
    .= t3`*`(t1`*`(t qua transformation of F,F1)) by A6,A5,Th6
    .= (t3 qua transformation of F2,F3)`*`(t1`*`t) by A1,A2,Def8
    .= t3 `*`(t1`*`t) by A3,A4,Def8;
end;
