14. Signal

  Where other programming languages have the "goto" command, Rexx has
  "signal".  The instruction "signal label" makes the program jump to
  the specified label (which is a name followed by a colon, the same as
  is used to start an internal subroutine or function).  However, once
  this has been done, it is not possible to resume any "select" or "do"
  control structures that have recently been entered.  Thus the main use
  of "signal" is to jump to an error handling routine when something goes
  wrong, so that the program can clean up and exit.

  There is a much more useful way of using "signal", however.  That is
  to trap certain kinds of error condition.  The conditions which may
  be trapped include: "syntax" (that is, any syntax error), "error" (any
  environment command that results in a non-zero exit code), "halt" (when
  the user interrrupts execution) and "novalue" (which is when a symbol is
  used without having been given a value).

  Error trapping for one of these conditions is turned on by

      signal on <condition>

  and is turned off by

      signal off <condition>

  When one of these conditions occurs, the program immediately signals to a
  label whose name is the same as that of the condition.  Trapping is turned
  off, so another "signal on" will be required if you want to continue to
  trap that condition.

  If you wish for the program to signal to a label whose name is not the
  same as that of the condition, then you may specify that label using the
  "name" parameter of a "signal on" instruction, like this:

      signal on syntax name my_label

  Whenever a "signal" occurs, the variable "sigl" is set to the line number
  of the instruction which caused the jump.  If the signal was due to an
  error trap, then the variable "rc" will be set to an error code.

      /* This program goes on forever until someone stops it. */
I     say "Press Control-C to halt"
O     say "Press Control-C or Control-Break to halt"
      signal on halt
      do i=1
         say i
         do 10000
         end
      end

      halt:
      say "Ouch!"
      say "Died at line" sigl