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 predefinedāalias'. 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.