replace existing effect of type `effect.this` by the new effect value `effect.this`.

This is a convenience feature for value type effects for which the type of
the effect instance equals the effect type. `ref` type effects typically have
values that may be children of the effect type that are of a different type, so
`effect_type.replace new_value` must be used.

replace may only be called during the execution of an operation of a currently instated
effect of the same effect type.

pre
  safety: effect.this.is_instated