Function: vecextract
Section: linear_algebra
C-Name: extract0
Prototype: GGDG
Help: vecextract(x,y,{z}): extraction of the components of the matrix or
 vector x according to y and z. If z is omitted, y designs columns, otherwise
 y corresponds to rows and z to columns. y and z can be vectors (of indices),
 strings (indicating ranges as in "1..10") or masks (integers whose binary
 representation indicates the indices to extract, from left to right 1, 2, 4,
 8, etc.)

