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DEBUG "Source DIRectories" Command...
SDIR [pathname [<integer>]]
If no options are given, this command lists the current source
directory pathnames, in the order they will be used.
When a pathname is given by itself, it is added to the end of the
list. If the optional <integer> is supplied, the pathname at that
position in the list is replaced with the new pathname. If the
integer is beyond the end, the integer is ignored.