Ast.Attr
val integrate : (string * string) -> attr -> attr
val union : attr -> attr -> attr
val set : string -> string -> attr -> attr
val add_class : string -> attr -> attr
val remove : string -> attr -> attr
val set_classes : string list -> attr -> attr
val put : (string * string) -> attr -> attr
val merge : attr -> attr -> attr
val apply_pending : attr -> attr -> attr