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diff --git a/subdirmk/cdeps.sd.mk b/subdirmk/cdeps.sd.mk
new file mode 100644
index 0000000..402e687
--- /dev/null
+++ b/subdirmk/cdeps.sd.mk
@@ -0,0 +1,24 @@
+# subdirmk - useful rules for making and using cpp .*.d files
+#  Copyright 2019 Mark Wooding
+#  Copyright 2019 Ian Jackson
+# SPDX-License-Identifier: LGPL-2.0-or-later
+
+&# Usage:
+&#   &:include subdirmk/cdeps.sd.mk
+&# (probably in Perdir.sd.mk)
+&#
+&# Arranges for automatic #include dependency tracking for
+&# C compilation.  The compiler is asked to write the dependencies to
+&#  .*.d and these are automatically included.
+&#
+&# There is a bug: if a #included file is deleted and all references
+&# in .c files to it removed, `make' will complain that it is needed
+&# and can't be built.  `make clean' will fix this.
+
+CDEPS_CFLAGS ?= -MD -MF $(*D)/.$(*F).d
+
+&DEPFILES += $(foreach b,$(patsubst %.o,%,$(&OBJECTS)), \
+		$(dir $b).$(notdir $b).d)
+-include $(&DEPFILES)
+
+&CLEAN += $(&DEPFILES)