abort the current execution and return from the surrounding call to `instate`.
NYI: CLEANUP: `return` is the same as `abort`. The term `return` seems common for algebraic effects, but is confusing since it is different to returning from a call. We need to decide to break with algebraic effect jargon (and remove `return`) or to stick with it (and rename `abort` as `return`).
Precondition
pre
safety: effect.this.is_instated
safety: effect.this.env.abortable
`instate`.
NYI: CLEANUP: `return` is the same as `abort`. The term `return` seems
common for algebraic effects, but is confusing since it is different to
returning from a call. We need to decide to break with algebraic effect
jargon (and remove `return`) or to stick with it (and rename `abort` as
`return`).