Download the actionscript3.syn
In the latest version of SEPY I’ve added a syntax checker (as many developers pray for), and now SEPY uses MTASC as engine for checking actionscript syntax.
You just need to download MTASC and set the binary path of the compiler in sepy preferences.
The testing version of sepy is available at sourceforge.
A useful feature of this integration is that, if an error is found, the file containing the error will be automatically opened in the exact position of the error.
P.S. Note that MTASC engine is slightly different than the Macromedia internal compiler. You can see the comparison in the mtasc homepage.