function s=subvar(s0,v0,v1)
 
  $Id: subvar.m,v 1.4 2003-01-08 03:46:16-06 brinkman Exp $
  (c) 2001, Peter Brinkmann (brinkman@math.uiuc.edu)
 
  subvar: substitutes a variable for another in an expression
 
  Usage example: subvar('sin(i)','i','x') yields 'sin(x)'
 
  Parameters:
 	s0: expression
 	v0: old variable
 	v1: new variable
 
  Returns:
 	s: s0 with all occurrences of v0 replaced by v1
 
  v0 must be a variable name. v1 can be almost anything, but the user is
  responsible for proper use of parentheses. For instance, if v1 is a
  sum, then it is wise to enclose it in parentheses. subvar will _not_ do
  this automatically.
 
  Note that subvar parses the expression s0 and only replaces occurrences
  of the _variable_ v0 by v1. In particular, it doesn't blindly substitute
  the string v1	for any substring v0.
 
  Although this function behaves much like Matlab's subs function, I had
  to reimplement this because (a) Octave has no such substitute function,
  and (b) the subs function automatically encloses v1 in parentheses,
  which is undesirable for the purposes of Iode.