anchor.diff:  
   statements : statement+ ;
   
  +/** A naked '=' op is not part of a valid expression so I'm making it a statement;
  + *  see comment on binary_expression.  It also resolves ambiguity between
  + *  rule pattern's expression_pattern alt and pattern_initializer.
  + */
  +assignment_statement
  + : try_operator? prefix_expression  assignment_operator try_operator? prefix_expression
  + ;
  +
   // GRAMMAR OF A LOOP STATEMENT
   
   loop_statement : for_statement
  
positive.code: 
  // GRAMMAR OF AN EXPRESSION PATTERN
  
  expression_pattern : expression  ;
  
  // Attributes
  
negative.code: constant_declaration : attributes? declaration_modifiers? 'let' pattern_initializer_list  ;
  pattern_initializer_list : pattern_initializer (',' pattern_initializer)* ;
  
  /** rule is ambiguous. can match "var x = 1" with x as pattern
   *  OR with x as expression_pattern.
   *  ANTLR resolves in favor or first choice: pattern is x, 1 is initializer.
   */
  pattern_initializer : pattern initializer? ;
  