First, there are many matrices in the resulting resolution coming from the res and nres commands. In order to use a specific matrix in the resolution, you can use indexing to refer to the matrix. Arbitrary integer expressions are allowed as the index. If the value of the index is out of range, then a matrix with zero rows and columns is returned. This is useful when writing scripts.
Indexing starts at one.
See section 7.2 for more details. The second special situation occurs when entering a polynomial expression. There are possible name conflicts between names of matrices and the names of the ring indeterminates. This problem occurs in several other places as well. .