If "!" is prefixed to a host,
it means that the host is excluded from the list.
All of elements in the list are scanned from the first one to the last one.
Thus once included (excluded) name may be excluded (included)
in succeeding list.
For example, for a HostList like follows:
a host named "host.yy.xx.dom" matches with the first hostSpec,
but excluded by the second one, but included again by the third one.
If the first host in a HostList is with "!", it means
exclusion from the universe ("*", that is any host), that is,
"!host, ..." is regarded as "*,!host, ..."