Dr David Flater, Dr Philippe Martin and Michelle L. Crane (2007).
Rendering UML Activity Diagrams as Human-Readable Text.
NISTIR report 7469, National Institute of Standards and Technology, Gaithersburg, MD, 2007.

HTML version:  on the NIST site,  on this site.
PDF version:  on the NIST site,  on this site.

List of the recent NIST publications (including this one).