theorem Th23: :: ALTCAT_4:23

for A, B being non empty transitive with_units AltCatStr
for F being contravariant Functor of A,B
for o1, o2 being Object of A
for a being Morphism of o1,o2 st <^o1,o2^> <> {} & <^o2,o1^> <> {} & a is coretraction holds
F . a is retraction