/**
   * Differentiates a variable. May want to alter behaviour when using multi equation as diff(f,x)
   * might not be zero.
   *
   * @return 1 if the variable has the same name as data, 0 if the variable has a different name.
   */
  public Object visit(ASTVarNode node, Object data) throws ParseException {
    String varName = (String) data;
    XVariable var = (XVariable) node.getVar();
    PartialDerivative deriv = null;
    if (var instanceof DVariable) {
      DVariable difvar = (DVariable) var;
      if (varName.equals(var.getName())) return nf.buildConstantNode(tu.getONE());
      else if (isConstantVar(var)) return nf.buildConstantNode(tu.getZERO());

      deriv = difvar.findDerivative(varName, localDJep);
    } else if (var instanceof PartialDerivative) {
      if (isConstantVar(var)) return nf.buildConstantNode(tu.getZERO());

      PartialDerivative pvar = (PartialDerivative) var;
      DVariable dvar = pvar.getRoot();
      deriv = dvar.findDerivative(pvar, varName, localDJep);

    } else throw new ParseException("Encountered non differentiable variable");

    Node eqn = deriv.getEquation();
    if (eqn instanceof ASTVarNode) return nf.buildVariableNode(((ASTVarNode) eqn).getVar());
    if (eqn instanceof ASTConstant) return nf.buildConstantNode(((ASTConstant) eqn).getValue());

    return nf.buildVariableNode(deriv);
  }
 public boolean isConstantVar(XVariable var) {
   if (!var.hasEquation()) return true;
   Node eqn = var.getEquation();
   if (eqn instanceof ASTConstant) return true;
   /* So why would we want a=x to be treated as a constant? */
   //		if(eqn instanceof ASTVarNode) {
   //			return isConstantVar((XVariable)((ASTVarNode) eqn).getVar());
   //		}
   return false;
 }