module type SYM_EXPR = sig

  module T:Sym_type.SYM_TYPE

  module R:Sym_ref.SYM_REF

  type t

  type repr

  type obs

  val value : 'a T.t -> t -> repr

  val ref : 'a R.t -> t -> repr

  val call0 : UTF8.t -> t -> repr

  val call1 : UTF8.t -> repr -> t -> repr

  val call2 : UTF8.t -> repr -> repr -> t -> repr

  val call3 : UTF8.t -> repr -> repr -> repr -> t -> repr

  val callN: UTF8.t -> repr list -> t -> repr

  val expression : repr -> t -> repr

  val observe : repr -> obs

end