use insert() inserts a nonzero coefficient at given coordinates row, col and returns its reference assuming that: 1 - the coefficient does not exist yet 2 - this the coefficient with greater inner coordinate for the given outer coordinate. In other words, assuming *this is column-major, then there must not exists any nonzero coefficient of coordinates ixcol such that i >= row. Otherwise the matrix is invalid.
use insert() Like fill() but with random inner coordinates. Compared to the generic coeffRef(), the unique limitation is that we assume the coefficient does not exist yet.