Lookaround = object
  ahead*: AheadSig
  behind*: BehindSig
  smL*: SmLookaround

func matchImpl(text: string; regex: Regex; m: var RegexMatch; start = 0): bool {.
    ...raises: [], tags: [RootEffect].}

func reversedMatchImpl(smA, smB: var Submatches; text: string; nfa: Nfa;
                       look: var Lookaround; start, limit: int): int {.
    ...raises: [], tags: [RootEffect].}

func startsWithImpl(text: string; regex: Regex; start: int): bool {....raises: [],
    tags: [RootEffect].}

template initLook(): Lookaround

template lookAroundTpl(): untyped {.dirty.}