To flesh out that idea even farther, the Include command should be recursive so that a phrogrammer does not have to remember dependancies.  It should also be distinct so that if a CircleClass and TriangleClass are both included, and they each contain an Include command pointing to the same TrigonometryClass, that the IDE will only end up loading the latter class once.  You might also give them their own file extention indicating that they are not full programs, and need not supply a Main routine.

 [Edited]  A suggested extension:     .PLF  Phrogram Library File