Permission is granted to anyone to use this software for any purpose,
including commercial applications, and to alter it and redistribute
it freely, subject to the following restrictions:

1. Redistributions of source code must retain the above copyright
    notice, disclaimer, and this list of conditions.
2. Redistributions in binary form must reproduce the above copyright
   notice, disclaimer, and this list of conditions in the documenta-
   tion and/or other materials provided with the distribution.
 3. All advertising materials mentioning features or use of this
    software must display the following acknowledgment:

This product includes software developed by Greg Roelofs
and contributors for the book, "PNG: The Definitive Guide,"
published by O'Reilly and Associates.