theorem :: NATTRA_1:34

for A, B being Category
for a, b being Object of (Functors (A,B))
for f being Morphism of a,b st Hom (a,b) <> {} holds
ex F, F1 being Functor of A,B ex t being natural_transformation of F,F1 st
( a = F & b = F1 & f = [[F,F1],t] )