<theory> The discriminated union of two sets A and B is
A + B = {(inA, a) | a in A} U {(inB, b)| b in B}
where inA and inB are arbitrary tags which specify which
summand an element originates from.
A type (especially an algebraic data type) might be described as a discriminated union if it is a sum type whose objects consist of a tag to say which part of the union they belong to and a value of the corresponding type.