nc_algebra: additional check needed

[levandov@…]

It turned out, that nc_algebra either does not check or checks incorrectly whether the nonzero elements in the first argument (structure constants) are invertible. For instance,

ring r = 0,(x,d),dp;
def R = nc_algebra(x^2,0); setring R;

should return an error since x2 is not invertible in r. Instead, just a commutative ring is returned. Tested with both 3-1-7 and 4-0-0. The problem applies to matrices in the first argument as well:

ring r=0,(x,y),dp;
matrix C[2][2]=0,x^2, 0,0;
def R = nc_algebra(C,0); setring R;

Notably, "R;" print commutative R but with the field "noncommutative relations:", that is the attempt of providing R with noncommutative structure has been tried.

Thanks in advance.

[Oleksandr]

i have simplified 2nd test.

[Oleksandr]

i am testing my fixes

[Oleksandr]

pushed as 5faa0e147458dacf2fbe178a3ebe9b2d0957424b