Explanation: The function UNION is used either to combine two different sets of the same type into a resulting set:
e.g. “The teacher asked all modelers and all handicraft enthusiasts of the school to prepare a contribution for the anniversary”;

or to add a single element having type n to an already existing set of type (n+1):
e.g. “All teachers and a student prepared the excursion.”

The latter case is needed for the representation of prepositions like “including”, of connectors like “and” or of adverbs like “additionally/in addition.”