Where to find technical documentation for the ABR file format?

I'm a programmer and would like to write a script that can load an ABR brushes file exported from photoshop, make some changes to it and write it to disk again.  I need a PDF or similar that describes the file format of this file.  I know these usedto be available, but can no longer locate any  with a web search.  Where could I find some technical documentation for the ABR file format?