# The beginning of the `snake lemma'.
category
snake
objects
A, B, C, A', B', C', Keh, Cof.
arrows
m:A->B, e:B->C, f:A->A', g:B->B', 
h:C->C', m':A'->B', e':B'->C', k:Keh->C, 
p:A'->Cof. 
relations
g*m = m'*f, h*e = e'*g.