anchor.diff:    : query 
     ;
   
  +axiom
  +  : EQ_TO expression -> ^(EQ_TO expression)
  +  | SC_OF expression -> ^(SC_OF expression)
  +  | expression
  +  ;
  +  
   expression
     : concept ('+' concept)* -> ^(TOP_AND ^(GENUS concept+)) 
     | concept ('+' concept)* (':' refinements) -> ^(TOP_AND ^(GENUS concept+) ^(DIFF refinements))
     ;
  -  
  +    
   concept
     : SCTID 
     | SCTID TERM -> ^(SCTID TERM)
  
positive.code:   | 'DescendantsAndSelf' '('  query ')' -> ^(DESC_SELF query)
    | 'Union' '(' querySet ')' -> ^(UNION querySet)
    | 'All' -> ^(ALL)
    | expression
    ;
    
  querySet
  
negative.code: DESC;
  DESC_SELF;
  ALL;
  EQ_TO;
  SC_OF;
  }
  
  @header {
  