Function: matintersect
Section: linear_algebra
C-Name: intersect
Prototype: GG
Help: matintersect(x,y): intersection of the vector spaces whose bases are
 the columns of x and y.
Doc: $x$ and $y$ being two matrices with the same number of rows, finds a
 basis of the vector space equal to the intersection of the spaces spanned by
 the columns of $x$ and $y$ respectively. For efficiency, the columns of $x$
 (resp.~$y$) should be independent.

 The faster function \tet{idealintersect} can be used to intersect
 fractional ideals (projective $\Z_{K}$ modules of rank $1$); the slower but
 more general function \tet{nfhnf} can be used to intersect general
 $\Z_{K}$-modules.
