anchor.diff:     : '+' NUMBER
      | '-' NUMBER
      | NUMBER
  +   ;
  +
  +VARIABLE
  +   : 'variable'
      ;
   
   IN
  
positive.code:    | MULTILINESTRING
     ;
  
  identifier
     : IDENTIFIER
     | 'variable'
     ;
  
  fragment DIGIT
     : [0-9]
     ;
  
negative.code:   ;
  
  variable
     : VARIABLE name blockbody
     ;
  
  block
  