the labelling of people can be related to a reference group. further, we say that f is   

left-exact if, whenever 0→a→b→c→0 is exact, then 0→f(a)→f(b)→f(c) is exact;
right-exact if, whenever 0→a→b→c→0 is exact, then f(a)→f(b)→f(c)→0 is exact;
half-exact if, whenever 0→a→b→c→0 is exact, then f(a)→f(b)→f(c) is exact. 347)
 the on-street parking (amendment no. this is particularly the case with large round bales; their size makes them difficult to flip, so it may not be feasible to flip many of them onto the flat surface for transport and then re-position them on the round surface at the destination. 