+;;; Calling-convention spec, typically one of predefined keywords.
+;;; Add or remove as needed for target platform. It makes sense to
+;;; support :cdecl everywhere.
+;;;
+;;; Null convention is supposed to be platform-specific most-universal
+;;; callout convention. For x86, SBCL calls foreign functions in a way
+;;; allowing them to be either stdcall or cdecl; null convention is
+;;; appropriate here, as it is for specifying callbacks that could be
+;;; accepted by foreign code both in cdecl and stdcall form.
+(def!type calling-convention () `(or null (member :stdcall :cdecl)))
+
+;;; Convention could be a values type class, stored at result-type.
+;;; However, it seems appropriate only for epilogue-related
+;;; conventions, those not influencing incoming arg passing.
+;;;
+;;; As of x86's :stdcall and :cdecl, supported by now, both are
+;;; epilogue-related, but future extensions (like :fastcall and
+;;; miscellaneous non-x86 stuff) might affect incoming argument
+;;; translation as well.
+