diff -r bb18daa62d5f -r b0ebb3d88407 src/cx/collection.h
--- a/src/cx/collection.h	Sat Dec 30 18:48:25 2023 +0100
+++ b/src/cx/collection.h	Sun Dec 31 14:29:46 2023 +0100
@@ -48,6 +48,8 @@
  */
 #define CX_STORE_POINTERS 0
 
+#ifndef CX_COMPARE_FUNC_DEFINED
+#define CX_COMPARE_FUNC_DEFINED
 /**
  * A comparator function comparing two collection elements.
  */
@@ -55,6 +57,7 @@
         void const *left,
         void const *right
 );
+#endif // CX_COMPARE_FUNC_DEFINED
 
 /**
  * Use this macro to declare common members for a collection structure.