Usage:
<from_bigraded r s m n segre
If m is a bigraded module over Pr x Ps then n is set to
the singly graded module over P^{rs+r+s} which
represents the same sheaf as m, via the segre embedding.
It is assumed that the current ring has at least
rs+r+s+1 variables, and the first rs+r+s+1 of these are
used.
Parameters:
r,s = positive integers
m = bigraded module over a polynomial ring in
two sets of variables: the first r+1 and the next
s+1. m must be generated by forms of bidgree d,d
for various d (that is, m must be a diagonal module.)
Output values:
n = module over the current ring.
segre = the segre map used
If R = k[zij] is the current ring, and S = k[ xi , yj ] is the
bihomogeneous base ring of m, then the desired module is the
R-submodule of m generated by the given S-generators, via the
segre map sending zij to xi yj.
If m is not generated by "diagonal" elements (bidegrees equal),
but, say by elements of bidegree d,e, then the result will
correspond to the module m, regraded -- that is, twisted by a line
bundel 0(d,e).