QUAIL

Solves triangular linear systems of the form xb = y or zb = y where z is the transpose of x. Returns b. By default, x is lower triangular and is not transposed. If transpose is non-nil, the transpose of x is used. If upper is non-nil, x is taken to be upper triangular. Elements of x outside of the triangular portion of interest are ASSUMED to be zero, regardless of their actual values.