In 3.09 we used to allow defining "infix" keywords by using 
antiquotations in inside the "EXTEND" and "DELETE_RULE" directives as 
follows:

--------------------------------------------------------------

(*
  * The prefix version of the infix expression
  *)
let prefix_name op =
    "prefix_" ^ op

(*
  * Add an infix keyword.
  *)
let add_infix (keyword : string) =
    EXTEND
       GLOBAL: expr;

       expr: LEVEL "expr1"
          ["expr1" LEFTA
           [ e1 = expr; op = $keyword$; e2 = expr ->
                <:expr< $lid:prefix_name op$ $e1$ $e2$ >>
           ]];
    END

(*
  * Remove the infix keyword.
  *)
let remove_infix (keyword : string) =
    DELETE_RULE
       expr:
          expr; $keyword$; expr
    END
--------------------------------------------------------------

This does not seem to be supported in 3.10. Is there some other way to 
achieve a similar effect in 3.10? Thanks!

Aleksey