special function message sent to objects by the 'contains' and 'is in' operators
built-in function to handle the containsItem message
When the contains or is in operator is used with an object to test whether the object contains a value, a containsItem message is sent to the object. The message is sent with the search string and a caseSensitive boolean as parameters. The object may handle the message and return True or False to indicate whether the object contains the search term. If it doesn’t handle the message, the built-in SenseTalk containsItem function is called, which performs the test as defined by the objectContainsItemDefinition global property.