anchor.diff:  
   // GRAMMAR OF A VARIABLE DECLARATION
   
  -variable_declaration : variable_declaration_head pattern_initializer_list
  +variable_declaration
  + : variable_declaration_head pattern_initializer_list
    | variable_declaration_head variable_name type_annotation code_block
    | variable_declaration_head variable_name type_annotation getter_setter_block
    | variable_declaration_head variable_name type_annotation getter_setter_keyword_block
  
positive.code: 
  // GRAMMAR OF A TYPE_CASTING OPERATOR
  
  type_casting_operator : 'is' type | 'as' '?'? type ;
  
  // GRAMMAR OF A POSTFIX EXPRESSION
  
  /*
  postfix_expression : primary_expression
   | postfix_expression postfix_operator
   | function_call_expression
   | initializer_expression
   | explicit_member_expression
   | postfix_self_expression
   | dynamic_type_expression
   | subscript_expression
   | forced_value_expression
   | optional_chaining_expression
   ;
  */
  
  // GRAMMAR OF A FUNCTION CALL EXPRESSION
  
  trailing_closure : closure_expression  ;
  
  // GRAMMAR OF A PRIMARY EXPRESSION
  
  
negative.code: 	|	expression '!'
  	|	expression '?'
  	;
  */
  
  // GRAMMAR OF A PREFIX EXPRESSION
  
  in_out_expression : '&' identifier ;
  
  binary_expression
    : binary_operator prefix_expression
    | assignment_operator prefix_expression
    | conditional_operator prefix_expression
    | type_casting_operator
    ;
  
  // GRAMMAR OF AN ASSIGNMENT OPERATOR
  
  assignment_operator : '='  ;
  