kernel

Usage:
    <kernel f0 a b c
Given f0, a, b making a commutative diagram
(where the vertical arrows go down):
   C1 --> A1 --> B1
   |      |      |
 c |     a|      | b
   |      |  f0  |
   C0 --> A0 --> B0
   |      |      |
   |      |  f   |
ker f --> A  --> B
we compute a presentation c for the kernel of f
Parameters:
            f0,a,b = matrices
Output values:
            c = matrix
We first compute the relations on the generators
of the image of f0 modulo the image of b,
then a presentation of the kernel is obtained
by computing the relations on this
modulo the image of the map presenting a.
User kernel_and_map to compute the map from the
kernel to A at the same time.