We do not allow the type of construct you have depicted. However, semantically the knotted attribute behaves just like the tie in your picture. The reasoning behind using an attribute is that a knot is normally thought of to contain a restricted set of values that attributes can use, also making a visual distinction between a knotted attribute and a knotted tie.
We have a refactoring rule, when you realize that whatever the knot represents actually has attributes of its own. The knotted attribute is replaced by a tie to an anchor with a single static attribute, after which you can add more attributes to that anchor. I have attached a picture:
Changing the cardinality of the tie should in this case also be able to capture the Set data type, but perhaps not as elegantly as if we would have allowed your construct.