Returns a collection containing all elements of self followed by object.

**Note**: at the time of writing, the OCL standard library sports a bug that changes *OrderedSets* in *Sets* when including elements.

Expression	Result
Sequence{'a', 'b'}->including('c')	Sequence{'a', 'b', 'c'}
Bag{'a', 'b'}->including('c')	Bag{'a', 'c', 'b'}
OrderedSet{'a', 'b'}->including('c')	Set{'a', 'c', 'b'}
Set{'a', 'b'}->including('c')	Set{'a', 'c', 'b'}