anchor.diff:  earlier : DotDot date;
   later : date DotDot;
   consecutives
  -    : YearMonthDay DotDot YearMonthDay
  -    | YearMonth DotDot YearMonth
  -    | Year DotDot Year;
  +    : yearMonthDay DotDot yearMonthDay
  +    | yearMonth DotDot yearMonth
  +    | year DotDot year;
   maskedPrecision : Digit Digit (Digit X | XX );
   
   // ********************  Level 2: Interval Parser Rules  ******************** //
  
positive.code: // ***********************   Level 0: Parser Rules   ************************ //
  
  level0Expression : date | dateTime | l0Interval;
  date : Year | YearMonth | YearMonthDay;
  dateTime : YearMonthDay T Time;
  
  // ********************  Level 0: Interval Parser Rules  ******************** //
  
  
negative.code: Colon : ':';
  Slash : '/';
  
  year : PositiveYear | NegativeYear | '0000';
  NegativeYear : Dash PositiveYear;
  PositiveYear
      : PositiveDigit Digit Digit Digit
  