preimage over ground rings: division by zero resp. segmentation fault

[seelisch]

The following code runs ok over Q but crashes over Z:

ring r1 = integer,(x,y,z),dp; ideal i1 = x2,y-z; ring r2 = integer,(a,b),dp; map m = r1,a,b,a+b; ideal i2 = m(i1); setring r1; preimage(r2, m, i2);

As a first step, enabling 'preimage' between polynomial rings over the SAME ground ring would be fine.

[seelisch]

fixed by git commit 729a6a812b03beeee8dc4bb0094b7a76db2e0d8c