general inverse solver?

Gabor G a ecrit:
Check out the Ryacas package.   There is a vignette with some
examples.

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Which led me to the manuals for yacas itself.  I'm guessing there may be 
a way to use yacas'  "AND" construct to combine a few equations and then 
hope the Newton Solver can work with that, but it's not clear that will 
work.

TK!Solver is nice because you aren't limited to linear equations, nor to 
equations which "fit" into a matrix structure, and because it's legal to 
have more than one unknown to be back-solved (assuming the problem is 
not under- or over-defined, of course).