Sys.VarVariables must be hashable, comparable, etc.
include Goblint_constraint.ConstrSys.SysVar with type t := tval is_write_only : t -> boolval pretty_trace : unit -> t -> GoblintCil.Pretty.docval printXml : 'a BatInnerIO.output -> t -> unitval var_id : t -> stringval node : t -> MyCFG.node