Having:
$ O \in \{{A,B,C}\} $
How to stress that $O$ has to be one of types defined in collection? Does inclusion already does it?
Having:
$ O \in \{{A,B,C}\} $
How to stress that $O$ has to be one of types defined in collection? Does inclusion already does it?
To say that $ x \in \{A, \, B, \, C \}$ already incorporates the idea that $x$ has to be exactly one of $A,\; B, $ or $C$.
Perhaps you wonder, how might one allow it to be both $A$ and $C$ ? One way of notating this would be to write $x \in \{A, \, B, \, C, \, \{A,\, C \} \, \}$, allowing $x$ to be the set containing both $A$ and $C$ (which is one way of notating that it's both).
If you really are just interested in stressing that it's exactly one of $A,\; B$ or $C$, you could just write that line out.