[web2py] SQLFORM.grid / appadmin: self-reference question

[lyn2py]

I have a table that self-references:

db.define_table('tags',
    Field('tag', 'string',length=255,requires=IS_NOT_EMPTY()),
    Field('description','text'),
    Field('parent_tags', 'list:reference tags'),
    format='%(tag)s'
)
And in appdamin and SQLFORM.grid, I noticed that there is a "+" sign next 
to the self-referenced field. I presumed, it may be used to add items to 
the field. But when I click on it, nothing happens.

Is this a bug? Or what is its intended behavior? Thanks!

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