diff --git a/libsrc/c16/isfast.s b/libsrc/c16/isfast.s
index ff104d97f..036840753 100644
--- a/libsrc/c16/isfast.s
+++ b/libsrc/c16/isfast.s
@@ -1,22 +1 @@
-;
-; Marco van den Heuvel, 2018-03-20
-;
-; unsigned char isfast (void);
-; /* Returns 1 if the CPU is in double clock mode. */
-;
-
-        .export         _isfast
-
-        .include        "plus4.inc"
-
-
-.proc   _isfast
-
-        lda     TED_CLK
-        lsr
-        and     #$01
-        ldx     #$00
-        rts
-
-.endproc
-
+.include "../plus4/isfast.s"