Library /sys$common/syshlp/helplib.hlb  —  CRTL  isatty  Return Values
    1                  If the file descriptor is associated with a
                       terminal.
    0                  If the file descriptor is not associated with
                       a terminal.
    -1                 Indicates an error (for example, if the file
                       descriptor is not associated with an open
                       file).
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