[web2py] How to modify the Multi Select Form in SQLFORM

[A E]

I have defined three tables

db.define_table('category_cat',
    Field('userinfo',db.auth_user,default=auth.user_id,
writable=False,readable=False),
    Field('name', requires=IS_NOT_EMPTY()))

db.define_table('category',
    Field('userinfo',db.auth_user,default=auth.user_id,
writable=False,readable=False),
    Field('category_cat',db.category_cat,requires=IS_IN_DB(db,
db.category_cat.id,'%(name)s')),
    Field('name', label='Sub Category', requires=IS_NOT_EMPTY()),
    Field('description', 'text'))

db.define_table('listing',
    Field('userinfo',db.auth_user,default=auth.user_id,
writable=False,readable=False),
    Field('category', db.category, label='Category * ~can select
multiple~', requires=IS_IN_DB(db,db.category.id,'%(name)s',multiple=True)),


I would like to display the category_cat entries in the Multi Select Form
Field (for the listings table) make them unselectable and group the
category entries under each one is this even possible? If so how can I
achieve this result

So for example in the Multi Select Form (listing table) I would like to have

category_cat 1 (not selectable)
   category 1
   category 2
category_cat 2 (not selectable)
   category 3
   category 4

but displaying the name value :-)

I hope I made sense and some one can help me :-)

Thank you

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