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4.3.2 The HALT condition
The HALT condition of external origin, which is raised as a result
of an action from the user, normally a combination of keys which
tries to abort the program. Which combination of keys will vary
between operating systems. Some systems might also simulate this
event by other means than key combinations. Consult system for
more information.

The differences between HALT and the standard condition are:

    The default-action for the HALT condition is to abort
  execution, as though a REXX runtime error number 4 (Program
  interrupted) had been reported. But note that SYNTAX will never be
  raised if HALT is not trapped.

    The delay-action of this condition is to ignore, not queue.

The standard allows the interpreter to limit the search for
situations that would set the HALT condition, to clause
boundaries. As a result, the response time from pressing the key
combination to actually raising the condition or triggering the
trap may vary, even if HALT is trapped by method SIGNAL. If a
clause for some reason has blocked execution, and never finish,
you may not be able to break the program.

The descriptive text returned by CONDITION() when called with the
Description option for condition HALT, is implementation
dependent, and may also be a nullstring. In general, it will
describe the way in which the interpreter was attempted halted, in
particular if there are more than one way to do raise a HALT
condition. Consult the implementation documentation for more