-The Muddle language is described in "The Muddle Programming Language",
-but in addition to the language itself, there is a rich and varied
-collection of software written in the language which facilitates the
-writing of programs and systems of programs in Muddle. The information
-describing this programming environment has been contained in various
-documents, some out of print or out of date, and in supplemental disk
-files describing changes and additions. Some of the packages of
-functions used to deal with Muddle code have never been formally
-documented. This manual brings together some of that scattered
-documentation.
+The Muddle language is described in "The Muddle Programming Language"
+[^2], but in addition to the language itself, there is a rich and
+varied collection of software written in the language which
+facilitates the writing of programs and systems of programs in Muddle.
+The information describing this programming environment has been
+contained in various documents, some out of print or out of date, and
+in supplemental disk files describing changes and additions. Some of
+the packages of functions used to deal with Muddle code have never
+been formally documented. This manual brings together some of that
+scattered documentation.
+
+[^2]: S. W. Galley and Greg Pfister.
+ *The Muddle Programming Language*.
+ MIT Laboratory for Computer Science, 1979.