AppleJavaExtensions.jar provides some stub methods to allow
compilation of code which contains Java methods that reference Mac OS
X specific Java APIs on any platform.  This is needed only for
compilation of the class edu.stanford.nlp.trees.tregex.gui.OSXAdapter .
Using this class and the links to Apple-specific technologies is
required to allow the Mac version of Tregex to behave like a normal
Mac application in responding to About and Preferences... menu items.

This library is not needed or used at runtime on any platform.

If you'd prefer not to have these complications in the source for your
use on other platforms, simply delete both AppleJavaExtensions.jar and
the file src/edu/stanford/nlp/trees/tregex/gui/OSXAdapter.java . The
OSXAdapter class is loaded using reflection by the main TregexGUI
class, so its absence will not cause any errors in compilation.

The file README-AppleJavaExtensions.txt contains Apple's README and
license information for AppleJavaExtensions.jar . More information on
AppleJavaExtensions can be found at:

  http://developer.apple.com/samplecode/AppleJavaExtensions/

This issue of needing to include AppleJavaExtensions.jar occurs for
many Java GUI programs which want to function well on Mac OS X,
including NetBeans, FindBugs, etc.  Do a Google search on:

  AppleJavaExtensions license

to find examples.