signal action siglist ?command?
              Specify  the  action  to take when a Unix signal is
              received by Extended Tcl, or a program that  embeds
              it.   Siglist  is  a list of either the symbolic or
              numeric Unix signal (the SIG prefix  is  optional).
              Action  is  one of the following actions to be per-
              formed on receipt of the signal.   To  specify  all
              modifiable  signals, use `*' (this will not include
              SIGKILL and SIGSTOP, as they can not be  modified).

              default - Perform system default action when signal
              is received (see signal system call documentation).

              ignore - Ignore the signal.

              error - Generate a catchable Tcl error.  It will be
              as if the command  that  was  running  returned  an
              error.  The error code will be in the form:
                  POSIX SIG signame
              For  the death of child signal, signame will always
              be SIGCHLD, rather than SIGCLD,  to  allow  writing
              portable code.

              trap  - When the signal occurs, execute command and
              continue execution if an error is not  returned  by
              command.   The  command  will  be  executed  in the
              global context.  The command will be edited  before
              execution,  replacing  occurrences of "%S" with the
              signal name.  Occurrences of "%%" result in a  sin-
              gle  "%".  This editing occurs just before the trap
              command is evaluated.  If  an  error  is  returned,
              then  follow  the  standard  Tcl  error  mechanism.
              Often command will just do an exit.

              get - Retrieve the current settings of  the  speci-
              fied  signals.   A keyed list will be returned were
              the keys are one of the specified signals  and  the
              values  are  a list cosisting of the action associ-
              ated with the signal, a 0  if  the  signal  may  be
              delivered (not block) and a 1 if it is blocked. The
              actions maybe one of `default',`ignore', `error' or
              `trap.  If the action is trap, the third element is
              the command associated with the action.  The action
              `unknown'  is  returned if a non-Tcl signal handler
              has been associated with the signal.

              set - Set signals from a keyed list in  the  format
              returned  by  the get.  For this action, siglist is
              the keyed list of signal state.   Signals  with  an
              action of `unknown' are not modified.

              block  -  Block  the  specified  signals from being
              received. (Posix systems only).
              unblock  -  Allow  the  specified  signal   to   be
              received.  Pending  signals  will not occur. (Posix
              systems only).

              The signal action will  remain  enabled  after  the
              specified  signal  has  occurred.  The exception to
              this is SIGCHLD on systems without  Posix  signals.
              For  these systems, SIGCHLD is not be automatically
              reenabled.  After a SIGCHLD signal is  received,  a
              call to wait must be performed to retrieve the exit
              status of the child process before issuing  another
              signal SIGCHLD ... command.  For code that is to be
              portable between both types of  systems,  use  this
              approach.

              Signals  are  not processed until after the comple-
              tion of the Tcl command that is executing when  the
              signal is received.  If an interactive Tcl shell is
              running, then the SIGINT will be set to error, non-
              interactive  Tcl  sessions  leave  SIGINT unchanged
              from when the process started (normally default for
              foreground  processes  and  ignore for processes in
              the background).

              This command is provided by Extended Tcl.