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The Scriptable Text Editor was a simple yet scriptable text editor that existed in AppleScript's early years. In older books on AppleScript you will often find reference to the Scriptable Text Editor as a means of manipulating text.
For some reason the text editors that have been included with Apple's operating systems, namely TeachText and now SimpleText, have been curiously unscriptable!
Fortunately, there are many excellent scriptable text editors out there, including Tex-Edit Plus, Mariner Write, and Microsoft Word. Scriptable Text Editor can be had here