dd -- differential in a chain complex

Description

dd -- a symbol used as a key in a chain complex, under which are stored the differentials.

The differentials are stored as the members in a chain complex map of degree -1. The map C.dd_i is the one whose source is the module C_i, and it is arranged to be zero if no map has been stored in that spot.

 i1 : R = ZZ/101[a,b]; i2 : C = resolution cokernel vars R 1 2 1 o2 = R <-- R <-- R <-- 0 0 1 2 3 o2 : ChainComplex i3 : C.dd 1 2 o3 = 0 : R <----------- R : 1 | a b | 2 1 1 : R <-------------- R : 2 {1} | -b | {1} | a | 1 2 : R <----- 0 : 3 0 o3 : ChainComplexMap i4 : C.dd_2 o4 = {1} | -b | {1} | a | 2 1 o4 : Matrix R <--- R