Replace the old node with the new one. 
    Old must be an indirect child of root
     
    :param root: ast node that contains an indirect reference to old
    :param old: node to replace
    :param new: node to replace `old` with

def replace_nodes(root, old, new):

    '''
    Replace the old node with the new one. 
    Old must be an indirect child of root
     
    :param root: ast node that contains an indirect reference to old
    :param old: node to replace
    :param new: node to replace `old` with 
    '''

    rep = Replacer(old, new)
    rep.visit(root)
    return