alarm seconds
          Instructs the system to send a SIGALRM  signal  in  the
          specified  number of seconds.  This is a floating point
          number, so fractions of a section may be specified.  If
          seconds is 0.0, any previous alarm request is canceled.
          Only one alarm at a time may  be  active;  the  command
          returns  the  number  of  seconds  left in the previous
          alarm.  On systems without the setitimer  system  call,
          seconds is rounded up to an integer number of seconds.

          The alarm command is not available on Windows 95/NT.

          This command is provided by Extended Tcl.