INCLUDE_IF_EXISTS
The keyword INCLUDE_IF_EXISTS is exactly like INCLUDE except that the included file does not have to exist in the "\Frontline\My Decoders\" directory. If the file is found, it will be included. If the file is not found, no error will be generated. Frontline widely employs this keyword to allow customers to optionally include their own DecoderScript in Frontline decoders. For more information on how to use this powerful feature, see "Extending Frontline Decoders".
For example:
INCLUDE_IF_EXISTS commandtable.dh
will include the file commandtable.dh if it exists.