# Module
  type selltable_key = (Product.t * Country.t)
  type selltable_value = Sell.t list
  type t
  ...
end

module Selltable = struct
  type selltable_key = (Product.t * Country.t)
  type selltable_value = Sell.t list
  type t = (selltable_value, selltable_key) Hashtbl.t
  ...
end

match Hashtbl.find_opt sells @@ get_product_key product with
    | None -> []
    | Some product_sells' -> ...
end

#Error
This expression has type
  (Matching_engine__Selltable.Selltable.selltable_key,
   Matching_engine__Selltable.Selltable.selltable_value)
  Hashtbl.t
but an expression was expected of type Matching_engine__Selltable.Selltable.t