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diff --git a/src/lib/cc/tokpaste.h b/src/lib/cc/tokpaste.h
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+/* Copyright (c) 2001 Matej Pfajfar.
+ * Copyright (c) 2001-2004, Roger Dingledine.
+ * Copyright (c) 2004-2006, Roger Dingledine, Nick Mathewson.
+ * Copyright (c) 2007-2020, The Tor Project, Inc. */
+/* See LICENSE for licensing information */
+
+/**
+ * @file tokpaste.h
+ * @brief Token-pasting macros.
+ **/
+
+#ifndef TOR_LIB_CC_TOKPASTE_H
+#define TOR_LIB_CC_TOKPASTE_H
+
+/**
+ * Concatenate `a` and `b` in a way that allows their result itself to be
+ * expanded by the preprocessor.
+ *
+ * Ordinarily you could just say `a ## b` in a macro definition.  But doing so
+ * results in a symbol which the preprocessor will not then expand.  If you
+ * wanted to use `a ## b` to create the name of a macro and have the
+ * preprocessor expand _that_ macro, you need to have another level of
+ * indirection, as this macro provides.
+ **/
+#define PASTE(a,b) PASTE__(a,b)
+
+/** Helper for PASTE(). */
+#define PASTE__(a,b) a ## b
+
+#endif /* !defined(TOR_LIB_CC_TOKPASTE_H) */