<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _is_defaultable(i, entry, table, check_for_aliases=True):
 """Determine if an entry may be removed from a routing table and be replaced by a default route. Parameters i : int Position of the entry in the table entry : RoutingTableEntry The entry itself The table containing the entry. check_for_aliases : bool If True, the table is checked for aliased entries before suggesting a route may be default routed. """

    # May only have one source and sink (which may not be None)
    if (len(entry.sources) == 1 and
            len(entry.route) == 1 and
            None not in entry.sources):
        # Neither the source nor sink may be a core
        source = next(iter(entry.sources))
        sink = next(iter(entry.route))

        if source.is_link and sink.is_link:
            # The source must be going in the same direction as the link
            if source.opposite is sink:
                # And the entry must not be aliased
                key, mask = entry.key, entry.mask
                if not check_for_aliases or \
                        not any(intersect(key, mask, d.key, d.mask) for
                                d in table[i+1:]):
                    return True
    return False