theorem Th19: :: ALTCAT_3:19

for C being category
for o1, o2, o3 being Object of C st <^o1,o2^> <> {} & <^o2,o3^> <> {} & <^o3,o1^> <> {} holds
for A being Morphism of o1,o2
for B being Morphism of o2,o3 st A is coretraction & B is coretraction holds
B * A is coretraction