| a matrix:
- the inverse matrix of A, if A is invertible;
- the 1x1 0-matrix if A is not invertible (in the polynomial ring!).
There are the following options:
- opt=0 or not given: heuristically best option from below
- opt=1 : apply std to (transpose(E,A)), ordering (C,dp).
- opt=2 : apply interred (transpose(E,A)), ordering (C,dp).
- opt=3 : apply lift(A,E), ordering (C,dp).
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