This is the same function as qr.solve, but it throws an error if an exact fit cannot
be found (instead of returning a least square fitted value, which is the default behavior
of qr.solve).
Usage
qr.solve_exact(a, b, tol = 1e-07)
Arguments
a
a QR decomposition or a rectangular matrix.
b
a vector or matrix of right-hand sides of equations.
incidence.matrix.
tol
the tolerance for detecting linear dependencies in the columns of x.
Only used if LAPACK is false and x is real.