def squarify(triangular_matrix: csr_matrix) -> csr_matrix:
"""Mirror a triangular matrix at the diagonal to make it a square matrix.
The input matrix *must* be upper triangular to begin with, otherwise
the results will be incorrect. No guard rails!
"""
assert (triangular_matrix.shape[0] == triangular_matrix.shape[1]
), "needs to be square matrix"
# The matrix is already upper diagonal. Use the transpose method, see
# https://stackoverflow.com/a/58806735/2340703.
return (triangular_matrix + triangular_matrix.T -
scipy.sparse.diags(triangular_matrix.diagonal()))
