anchor.diff:                        M     { $type = EMS;          }
                       | X     { $type = EXS;          }
                   )
  +            | (R E M) =>
  +                R E M       { $type = REM;          }
               | (P(X|T|C))=>
                   P
                   (
  
positive.code:             | LENGTH
              | EMS
              | EXS
              | ANGLE
              | TIME
              | FREQ
  
negative.code: //          number parsing rule.
  //
  fragment    EMS         :;  // 'em'
  fragment    REMS        :;  // 'rem'
  fragment    EXS         :;  // 'ex'
  fragment    LENGTH      :;  // 'px'. 'cm', 'mm', 'in'. 'pt', 'pc'
  fragment    ANGLE       :;  // 'deg', 'rad', 'grad'
  