TYPE is not a collection! But conceptually it is a SET, set of all its possible instances. quot: "Types and Programming Languages (2002)(Benjamin C. Pierce)" [page 92] Chapter 8 Typed Arithmetic Expressions "a term t has type T" (or "t belongs to T," or "t is an element of T") An object obj has a type T or obj is a type T <==> obj belongs to T So, "obj in T" is fine. e.g. "1 in int" means 1 is in the whole number set.

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Note that TYPE is SET; What does that mean? I don't understand.

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obj in T; But obj is **not** in T, since T is a type, not a container.

"is-a" tests are not the same as "in" tests. They are completely unrelated comparisons.