@staticmethod
def follow_expr(expr, _, nofollow, follow_mem=False, follow_call=False):
    """Returns True if we must visit sub expressions.
        @expr: expression to browse
        @follow: set of nodes to follow
        @nofollow: set of nodes not to follow
        @follow_mem: force the visit of memory sub expressions
        @follow_call: force the visit of call sub expressions
        """
    if not follow_mem and expr.is_mem():
        nofollow.add(expr)
        return False
    if not follow_call and expr.is_function_call():
        nofollow.add(expr)
        return False
    return True
