Why does this not work ? There is a simple integer solution.
(%i1) load("mnewton")$
(%i2) mnewton([a*b+c+d-3, b*c+a+d-5, c*d+a+b-2, a*d+b+c-6], [a,b,c,d], [1,1,1,1]);
Unable to compute the LU factorization
#0: solve_by_lu(
Maxima encountered a Lisp error:
Error in MERROR [or a callee]: NIL is not of type CHARACTER.
Automatically continuing.
To enable the Lisp debugger set *debugger-hook* to nil.
(%i3)