contains is used to check a list of values, which is given on the left of the operator, against a single value on the right.



A person can have multiple nationalities. If you want to include US tax documents if one of those nationalities is as a citizen of the United States, attach this condition to those documents:

p is Person
p.nationalities contains us

The list of nationalities of the person in this case should include us , meaning this condition evaluates to true and the documents to which it is attached will be included in the case.