anchor.diff:  
   // GRAMMAR OF A PARENTHESIZED EXPRESSION
   
  -parenthesized_expression : '(' expression_element_list? ')'  ;
  -expression_element_list : expression_element (',' expression_element)* ;
  -expression_element : expression | identifier ':' expression  ;
  +parenthesized_expression : '(' expression ')'  ;
  +
  +// GRAMMAR OF A TUPLE EXPRESSION
  +
  +tuple_expression
  + : '(' ')'
  + | '(' tuple_element ',' tuple_element_list ')'
  + ;
  +
  +tuple_element_list : tuple_element (',' tuple_element)* ;
  +
  +tuple_element : expression | identifier ':' expression ;
   
   // GRAMMAR OF A WILDCARD EXPRESSION
   
  
positive.code: 
  // GRAMMAR OF A BINARY EXPRESSION
  
  
  binary_expression
    : binary_operator prefix_expression
  // as far as I can tell, assignment is not a valid operator as it has no return type
  
negative.code:  | superclass_expression
   | closure_expression
   | parenthesized_expression
   | tuple_expression
   | implicit_member_expression
   | wildcard_expression
   | selector_expression
   | key_path_expression
   ;
  
  implicit_member_expression : '.' identifier ;
  