'Returns a canonicalized index into a matrix.
Parameters
matrix : LinOp
The matrix to be indexed.
constraints : list
A list of constraints to append to.
row : int
The row index.
col : int
The column index.'

@staticmethod
def get_index(matrix, constraints, row, col):

    key = (ku.index_to_slice(row), ku.index_to_slice(col))
    (idx, idx_constr) = index.graph_implementation([matrix], (1, 1), [key])
    constraints += idx_constr
    return idx
