D)oors:  The "D" command takes you to the Door System.  "Doors" are
applications external to RBBS-PC that a SYSOP sets up so that users
who call in can exit to them and then return to RBBS-PC.  RBBS-PC
only acts as a "gateway" to a "door."

A "door" can be any application written in any language (i.e. on-
line ordering, on-line games, on-line data bases, etc.).

Because the input and output is via the communications port such
applications are restricted to a line-at-a-time input and output.

Some SYSOP's have "doors" that move the caller's cursor -- but
this requires the caller's communication software support the same
conventions for moving the cursor around on the screen as the
particular "door" that was invoked follows (there is no standard one).
