Sets a user defined attribute without a value. This is useful when you just want to put a 'mark' attribute. Its value is set to {@link bool#TRUE}.
@param key the key of the attribute @return The old value of the attribute. <tt>null</tt> if it is new.
See Implementation
Sets a user defined attribute without a value. This is useful when you just want to put a 'mark' attribute. Its value is set to {@link bool#TRUE}.
@param key the key of the attribute @return The old value of the attribute. <tt>null</tt> if it is new.