Indexing Functions

at	Update operations for read-only arrays.
diag_indices	Return the indices to access the main diagonal of an array.
tril_indices	Return the indices of the lower triangle of an (n, m) array.
triu_indices	Return the indices of the upper triangle of an (n, m) array.
unravel_index	Convert a flat index or array of flat indices into a tuple of coordinate arrays.