Cron = object
  fields: Table[FieldKind, Field]

proc getNextForLastDayOfMonth(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [], tags: [].}

proc getNextForLastDayOfWeek(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [], tags: [].}

proc getNextForIndex(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.raises: [],
    tags: [].}

proc getNextForRange(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.raises: [],
    tags: [].}

proc getNextForStepRange(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [], tags: [].}

proc getNextForStepNum(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [], tags: [].}

proc getNextForAll(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.raises: [],
    tags: [].}

proc getNextForStepAll(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [], tags: [].}

proc getNextForNum(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.raises: [],
    tags: [].}

proc getNextForSeq(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [Exception, UnpackError], tags: [RootEffect].}

proc getMinIter(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.raises: [],
    tags: [].}

proc getNext(expr: Expr; field: Field; dt: DateTime): Option[int] {...}{.
    raises: [Exception, UnpackError], tags: [RootEffect].}

proc getNext(field: Field; dt: DateTime): Option[int] {...}{.
    raises: [Exception, UnpackError], tags: [RootEffect].}

proc newCron(second: string = "*"; minute: string = "*"; hour: string = "*";
            day_of_month: string = "*"; day_of_week: string = "*"; month: string = "*";
            year: string = "*"): Cron {...}{.raises: [ValueError], tags: [].}

proc getNext(cron: Cron; dt: DateTime): Option[DateTime] {...}{.
    raises: [Exception, UnpackError, KeyError], tags: [RootEffect].}