omething, and there are no top-level rigid
symbol clashes.  I think of this as a potential unifier, because
he caller still has to worry about recursive unification of subterms.
<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 corresponding to combo[].
<LI> start_flag -