Support S7 extension of S4 classes and vice-versa

.Rbuildignore

@@ -22,3 +22,4 @@
 ^[.]?air[.]toml$
 ^\.claude$
 ^\.git-blame-ignore-revs$
+^\.lintr\.R$

.lintr.R

@@ -0,0 +1,6 @@
+linters <- lintr::linters_with_defaults(
+  object_name_linter = lintr::object_name_linter(
+    styles = "symbols",
+    regexes = c(symbols = "^(?:\\.)?[[:alnum:]]+(?:[._][[:alnum:]]+)*$")
+  )
+)

NAMESPACE

@@ -37,6 +37,7 @@ export("method<-")
 export("prop<-")
 export("props<-")
 export(S4_register)
+export(S4_register_contains)
 export(S7_class)
 export(S7_class_desc)
 export(S7_data)