anchor.diff:  
   command
     :  lvalue_list ('=' expr_list)?
  +  |  'continue'
  +  |  'break'
  +  |  'return'
  +  |  'throw'
     ;
   
   lvalue_list
  
positive.code:   |  INT
    |  FLOAT
    |  STRING
    |  '[' arglist ']'
    |  '(' expr ')'
    ;
    
  // FIX: != could be wrong
  // var a:Type!=Type!(thing)
  mode
    :  '~' | '!' | '@' | '[:' expr ']'
    ;
  
  formalargs
  
negative.code:   |  do_loop ';'
    |  match
    |  command ';'
    ;
  
  block
  