Nilability = enum
  Parent, Safe, MaybeNil, Nil, Unreachable

func `$`(a: ExprIndex): string {....raises: [], tags: [].}

func `$`(a: SetIndex): string {....raises: [], tags: [].}

func `+`(a: ExprIndex; b: ExprIndex): ExprIndex {....raises: [], tags: [].}

func `+`(a: SetIndex; b: SetIndex): SetIndex {....raises: [], tags: [].}

func `-`(a: ExprIndex; b: ExprIndex): ExprIndex {....raises: [], tags: [].}

func `-`(a: SetIndex; b: SetIndex): SetIndex {....raises: [], tags: [].}

func `<=`(a: ExprIndex; b: ExprIndex): bool {....raises: [], tags: [].}

func `<`(a: ExprIndex; b: ExprIndex): bool {....raises: [], tags: [].}

func `==`(a: ExprIndex; b: ExprIndex): bool {....raises: [], tags: [].}

func `==`(a: SetIndex; b: SetIndex): bool {....raises: [], tags: [].}

func `>=`(a: ExprIndex; b: ExprIndex): bool {....raises: [], tags: [].}

func `>`(a: ExprIndex; b: ExprIndex): bool {....raises: [], tags: [].}

proc add[T, U](a: var SeqOfDistinct[T, U]; value: U)

proc checkNil(s: PSym; body: PNode; conf: ConfigRef; idgen: IdGenerator) {....raises: [
    KeyError, Exception, ValueError, OSError, IOError, ERecoverableError], tags: [
    RootEffect, ReadDirEffect, WriteIOEffect, ReadIOEffect, ReadEnvEffect].}