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diophantine equation given by ab, m, n.  It uses Huet's algorithm
(Information Processing Letters 7(3) 1978).
<P>
The equation has the form a1x1 + ... + amxm  =  b1y1 + ... + bnyn.
<UL>
<LI> ab[] - the vector of coefficients.  1..m is a1..am,  m+1..m+n is b1..bn.
<LI> m, n - the number of a and b coefficients.
<LI> constraints[] - rigid symbol (constant/function)restrictions.
<LI> basis[][] (output) - vector of minimal solutions.
<LI> num_basis (output) - number of minimal solutions.
</UL>
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