Library /sys$common/syshlp/dbg$help.hlb DEBUG, CANCEL, SOURCE, Qualifiers, /EDIT  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Cancels the effect of a previous SET SOURCE/EDIT command, which
    specifies the directory search list to be used during execution
    of the debugger's EDIT command. Canceling this command means the
    debugger searches for a source file in the directory in which it
    was compiled.