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 around on the
screen rather than just scroll each line away.   This full screen mode
requires that the caller's communication package support the same
conventions for moving the cursor around on the screen as the particular
"door" that was invoked follows.  Many packages support a "graphics" or
"ANSI" mode that is commonly used.
