code: public void setRoot(int context, Object environment)
    {
    	super.setRoot(context, environment);
    	m_exprObj = executeFilterExpr(context, m_execContext, getPrefixResolver(), 
    	                  getIsTopLevel(), m_stackFrame, m_expr);
    }
docs: Initialize the context values for this expression
  after it is cloned.
  
  @param context The XPath runtime context for this
  transformation.