anchor.diff:  
   primaryExpression
       : literal                                   # literalExpression
  -    | IDENTIFIER                                # identifierExpression    
  +    | IDENTIFIER                                # lookupExpression
       | '(' expression ')'                        # parenthesizedExpression
       | primaryExpression '.' IDENTIFIER          # memberAccessExpression        
       | primaryExpression '(' argumentList ')'    # invocationExpression
  
positive.code:     ;
  
  statementExpression
      : primaryExpression '(' argumentList ')'
      | assignmentExpression
      ;
  
  
negative.code:     ;
  
  literal
      : boolLiteral
      | numberLiteral
      | stringLiteral
      | nullLiteral
      ;
  
  boolLiteral
      : TRUE
      | FALSE
      ;
  
  numberLiteral
      : NUMBER
      ;
  
  stringLiteral
      : STRING
      ;
  
  nullLiteral
      : NULL
      ;
  
  VAR: 'var';
  TRUE: 'true';
  FALSE: 'false';
  
  NUMBER
      : DECIMAL_DIGITS '.' DECIMAL_DIGITS EXPONENT?
  