I skimmed the specs and found this on p. 111:
<property> ::= [<visibility>] [/] <name> [: <prop-type>] [[ <multiplicity-range> ]] [= <default>] [{ <prop-modifier > [, <prop-modifier >]* }]
(see also below in the specs). So I altered he Union and IsConst drop downs which showed {readOnly,union}. I couldn't find the bag, except you mean the foo bar here xD
q.