Define the set of CHR ports on which the CHR tracer asks for user
intervention (i.e. stops). Spec is either a list of ports as
defined in section 9.4.1
or a predefined `alias'. Defined aliases are: full to stop
at all ports, none or off to never stop, and default
to stop at the call,
exit, fail, wake and apply
ports. See also leash/1.