BatNumber.RefOpsReference operators ala C. Mutates a reference value. x -= y is the same as x := !x - y. 
val (+=) : bat__refops_t ref -> bat__refops_t -> unitval (-=) : bat__refops_t ref -> bat__refops_t -> unitval (*=) : bat__refops_t ref -> bat__refops_t -> unitval (/=) : bat__refops_t ref -> bat__refops_t -> unit