anchor.diff:  prefix_arg :
        pow_arg
     |  t_NOT t_FILLER? prefix_arg
  -  |  t_PLUS t_FILLER? prefix_arg
  -  |  t_MINUS t_FILLER? prefix_arg
  +  |  (t_PLUS | t_MINUS) t_FILLER? prefix_arg //NB: plus_arg may break if this is written as two rules
     ;
   
   pow_arg :
  
positive.code: prefix_expr :
       pow_expr
    |  t_NOT t_FILLER? prefix_expr
    |  t_PLUS t_FILLER? prefix_expr
    |  t_MINUS t_FILLER? prefix_expr
    ;
  
  pow_expr :
  
negative.code:   ;
  
  element :
       expr
    ;
  
  //non-empty
  