Function: matsupplement
Section: linear_algebra
C-Name: suppl
Prototype: G
Help: matsupplement(x): supplement the columns of the matrix x to an
 invertible matrix.
Doc: assuming that the columns of the matrix $x$
 are linearly independent (if they are not, an error message is issued), finds
 a square invertible matrix whose first columns are the columns of $x$,
 i.e.~supplement the columns of $x$ to a basis of the whole space.
 \bprog
 ? matsupplement([1;2])
 %1 =
 [1 0]

 [2 1]
 @eprog
 Raises an error if $x$ has 0 columns, since (due to a long standing design
 bug), the dimension of the ambient space (the number of rows) is unknown in
 this case:
 \bprog
 ? matsupplement(matrix(2,0))
   ***   at top-level: matsupplement(matrix
   ***                 ^--------------------
   *** matsupplement: sorry, suppl [empty matrix] is not yet implemented.
 @eprog
