Module type `PrettyPrintable.PPUniqRankSet`

set where at most one element of a given rank can be present

val add : t -> elt -> t
val empty : t
val equal : t -> t -> bool
val find_rank : t -> rank -> elt option
val fold : t -> init:'accum -> f:('accum -> elt -> 'accum) -> 'accum
val fold_map : t -> init:'accum -> f:('accum -> elt -> 'accum * elt) -> 'accum * t
val is_empty : t -> bool
val is_singleton : t -> bool
val is_subset : t -> of_:t -> bool
val map : t -> f:(elt -> elt) -> t
val singleton : elt -> t
val elements : t -> elt list
val remove : elt -> t -> t
val mem : elt -> t -> bool
val union_prefer_left : t -> t -> t: in case an element with the same rank is present both in lhs and rhs, keep the one from lhs in union_prefer_left lhs rhs