Pairing_heap.Eltval sexp_of_t : ('a -> Ppx_sexp_conv_lib.Sexp.t) -> 'a t -> Ppx_sexp_conv_lib.Sexp.tval value_exn : 'a t -> 'avalue_exn t returns the value in the heap controlled by this token if the value is still in the heap, and raises otherwise.