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3.4 Special Types and Type Constructors

3.4.3 Least-Upper-Bound Types

The least upper bound of two types in the type lattice is notated with the following syntax:

lub_type	::=	type "|" type

The type type1 | type2 is a supertype of both type1 and type2, and a subtype of all types that are supertypes of both type1 and type2. Least-upper-bound types are most useful in conjunction with parameterized types, described in section 4.