List operator and literal mutations:
++↔--(list concatenation ↔ difference)- a non-empty list literal →
[]
Not mutated: on the RHS of a guard in (when x in [a, b]) the [] collapse yields x in [] ≡ false, which Mutare.Mutators.Conditional already produces — so it is dropped there (the list's elements still mutate). Body in expressions keep the collapse because left-side evaluation is observable.
Filterable variants — qualify a # mutare:ignore filter with :label to suppress just one kind (Mutare.Mutator.variants/0): ++, --, empty.