MeatAxe matrices behave similar to lists of lists that are regarded as
matrices by GAP, e.g., there are functions like Rank or Transposed
that work for both types, and one can multiply or add two MeatAxe
matrices, the result being again a MeatAxe matrix.
But one cannot access rows or single entries of a MeatAxe matrix mat,
for example mat[1] will cause an error message.
MeatAxe matrices are constructed or notified by MeatAxeMat MeatAxeMat.
IsMeatAxeMat( obj )
returns true if obj is a MeatAxe matrix, and false otherwise.
GAP 3.4.4