Versions 6.7, 6.8, 2003

Here is a simple method that will return as the function result the directory separator for the local platform:

C_STRING(2;$0)
C_LONGINT($platform)
PLATFORM PROPERTIES($platform)
If ($platform=3)
     $0:="\\" ` use single backslash (\) for all 6.x versions
Else
     $0:=":"
End if