anchor.diff:    | 'DescendantsAndSelf' '('  query ')' -> ^(DESC_SELF query)
     | 'Union' '(' querySet ')' -> ^(UNION querySet)
     | 'All' -> ^(ALL)
  -  | expression
  +  | axiom
     ;
     
   querySet
  
positive.code: SCTID 
    : '-'? NONZERO DIGIT*;
  
  fragment DIGIT
    : '0'..'9'
    ;
  
negative.code: DESC;
  DESC_SELF;
  ALL;
  EQ_TO;
  SC_OF;
  }
  
  @header {
  