block_map

Usage:
    <block_map lo hi f
Create a ring map (over the current ring) which sends the given range 
of variables to themselves, but sends every other variable to 0.
Parameters:
  lo, hi = two integers specifying the range of variables.  The variables
       are numbered starting at 1.
Output values:
  f = 1xn matrix which is the desired ring map.
Caveats: if lo <= 0, problems can occur.