, in combos[] during the first call,
up to MAX_COMBOS subsets satisfying the restriction, then
give them out in the subsequent calls.  The first bunch of
parameters are the same as for the other next_combo routines.
<UL>
<LI> length -- total number of coefficients (m+n).
<LI> basis[][] -- basis of solutions.
<LI> num_basis -- number of basis solutions.
<LI> constraints[] -- rigid symbol constraints.
<LI> combo[] (input and output) -- current subset of the basis.
<LI> sum[]   (intput and output) -- sum corre