let A, B be category; :: thesis: for F, F1, F2 being covariant Functor of A,B st F is_transformable_to F1 & F1 is_transformable_to F2 holds
for t1 being transformation of F,F1 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t1 ! b) * (F . f) = (F1 . f) * (t1 ! a) ) holds
for t2 being transformation of F1,F2 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) holds
for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)

let F, F1, F2 be covariant Functor of A,B; :: thesis: ( F is_transformable_to F1 & F1 is_transformable_to F2 implies for t1 being transformation of F,F1 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t1 ! b) * (F . f) = (F1 . f) * (t1 ! a) ) holds
for t2 being transformation of F1,F2 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) holds
for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a) )

assume that
A1: F is_transformable_to F1 and
A2: F1 is_transformable_to F2 ; :: thesis: for t1 being transformation of F,F1 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t1 ! b) * (F . f) = (F1 . f) * (t1 ! a) ) holds
for t2 being transformation of F1,F2 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) holds
for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)

let t1 be transformation of F,F1; :: thesis: ( ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t1 ! b) * (F . f) = (F1 . f) * (t1 ! a) ) implies for t2 being transformation of F1,F2 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) holds
for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a) )

assume A3: for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t1 ! b) * (F . f) = (F1 . f) * (t1 ! a) ; :: thesis: for t2 being transformation of F1,F2 st ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) holds
for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)

let t2 be transformation of F1,F2; :: thesis: ( ( for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ) implies for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a) )

assume A4: for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds (t2 ! b) * (F1 . f) = (F2 . f) * (t2 ! a) ; :: thesis: for a, b being Object of A st <^a,b^> <> {} holds
for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)

let a, b be Object of A; :: thesis: ( <^a,b^> <> {} implies for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a) )
A5: <^(F . b),(F1 . b)^> <> {} by A1;
A6: <^(F . a),(F1 . a)^> <> {} by A1;
A7: <^(F1 . b),(F2 . b)^> <> {} by A2;
A8: <^(F1 . a),(F2 . a)^> <> {} by A2;
assume A9: <^a,b^> <> {} ; :: thesis: for f being Morphism of a,b holds ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)
then A10: <^(F . a),(F . b)^> <> {} by FUNCTOR0:def 18;
let f be Morphism of a,b; :: thesis: ((t2 `*` t1) ! b) * (F . f) = (F2 . f) * ((t2 `*` t1) ! a)
A11: <^(F2 . a),(F2 . b)^> <> {} by ;
A12: <^(F1 . a),(F1 . b)^> <> {} by ;
thus ((t2 `*` t1) ! b) * (F . f) = ((t2 ! b) * (t1 ! b)) * (F . f) by A1, A2, Def5
.= (t2 ! b) * ((t1 ! b) * (F . f)) by
.= (t2 ! b) * ((F1 . f) * (t1 ! a)) by A3, A9
.= ((t2 ! b) * (F1 . f)) * (t1 ! a) by
.= ((F2 . f) * (t2 ! a)) * (t1 ! a) by A4, A9
.= (F2 . f) * ((t2 ! a) * (t1 ! a)) by
.= (F2 . f) * ((t2 `*` t1) ! a) by A1, A2, Def5 ; :: thesis: verum