anchor.diff:  //          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'
  
positive.code:                       M     { $type = EMS;          }
                      | X     { $type = EXS;          }
                  )
              | (P(X|T|C))=>
                  P
                  (
  
negative.code:             | LENGTH
              | EMS
              | EXS
              | REMS
              | ANGLE
              | TIME
              | FREQ
  