anchor.diff:  
   maybe_cmd options{ backtrack=true; } :
        expr (t_FILLER? t_ASSIGN t_FILLER? expr)?
  -  |  t_IDENTIFIER cmd_args
  +  //|  t_IDENTIFIER cmd_args //TODO-AC: re-enable once I figure out side-effects/backtracking
     ;
   
   cmd_args :
  
positive.code:   ;
  
  element :
       expr_in_array
    ;
  
  //non-empty
  
negative.code:   |  matrix
    |  cell_array
    |  access
    |  t_AT t_FILLER? name
    ;
  
  