Usage:
<pts_linear I M J
Find a linear subsystem of the polynomials I vanishing at the
points which are the rows of M.
Parameters:
I = ideal of polynomials all of the same degree.
M = m by n matrix with constant entries, such that the i th row
represents a point in P^{n-1}, where n = number of variables of the
base ring.
Output values:
J = ideal consisting of the sub-vector space of I vanishing at every
point in M.