anchor.diff:     | tagcase_stmt
      | tagtest_stmt
      | tagwait_stmt
  -   | 'abort' 'return' (expression (',' expression)*)?
  -   | 'yield' (expression (',' expression)*)*
  +   | 'abort'? 'return' (expression (',' expression)*)?
  +   | 'yield' (expression (',' expression)*)?
      | 'abort'? 'signal' name (expression (',' expression)*)?
      | 'abort'? 'exit' name (expression (',' expression)*)?
      | 'abort'? 'break'
  
positive.code:    ;
  
  IDENTIFIER
     : [a-zA-Z] [a-zA-Z0-9]+
     ;
  
  DIGIT
     : [0-9]
     ;
  
  
negative.code:    ;
  
  for_stmt
     : 'for' decl (',' decl)* 'in' call 'do' body 'end'
     | 'for' idn (',' idn)* 'in' call 'do' body 'end'
     ;
  
  if_stmt
  