Index: doc/bibliography/pl.bib
===================================================================
--- doc/bibliography/pl.bib	(revision 2c065ed9943f494d509d8731145987742b50984e)
+++ doc/bibliography/pl.bib	(revision e25b30b3ad46cfd2b5d01193b1795e6fdb8d9b63)
@@ -5214,4 +5214,14 @@
 % M
 
+@misc{M4,
+    keywords	= {macros, preprocessor},
+    contributer	= {pabuhr@plg},
+    author	= {Brian W. Kernighan and Dennis M. Ritchie},
+    title	= {The M4 Macro Processor},
+    year	= 1977,
+    howpublished= {\url{https://wolfram.schneider.org/bsd/7thEdManVol2/m4/m4.pdf}},
+    optnote	= {Accessed: 2016-09},
+}
+
 @book{M68K,
     keywords	= {M680XX, Motorola},