Fixtype of modes.
This is a tagged union type, introduced by fty::deftagsum.
[Yul: Specification of Yul: Formal Specification] introduces the notion of mode, which indicates how a statement completes execution.
We disable the executable counterparts of the constructors of this type, because we want to treat them abstractly. Their definition is already disabled by default, as it should, but the fact that their executable counterpart is enabled defeats the disabling of their definition, given that they are nullary.