??so given (a,b,c,d,e) with a and e N-sets, c a Z-set, b "N-equivariant from a to c along N included into Z as the negatives" and d "N-equivariant from e to c along N included into Z as the positives" ...

??something about ... ????comonad here .... ??"replace c by the pullback of ..." ... ????or something ?? ... ??not quite right??...

???maybe also... ???replace a and e by pullbacks ... ????or something ???....