(** The builtin relations every world carries.

    Builtins are relations with operational capabilities beyond
    scanning: they check membership against the host (the domain
    relations), evaluate and solve (arithmetic), unify (equality),
    and generate boundedly (Between). The VM consults this module
    before the world.

    A builtin never enumerates an infinite domain. When its
    arguments are too unbound to decide, the answer is {!Stuck}, and
    the planner tries it again later or rejects the plan; totality
    is preserved either way. *)

type outcome =
  | Solved of Pattern.Subst.t list
      (** Zero or more ways to satisfy the goal. The empty list is
          failure. *)
  | Stuck
      (** Not decidable at the current bindings; retry once more
          variables are ground. *)

val is_builtin : string -> bool
(** Whether a name is a builtin. The loader uses it for reference
    checking, so user relations cannot shadow builtins. *)

val run : Pattern.Subst.t -> string -> Pattern.t Label.Map.t -> outcome option
(** Executes one builtin goal under the substitution. [None] when
    the name is not a builtin, so the VM falls through to the
    world. *)