The expression consists of one or more primitives. Primitives usually consist of an id (name or number) preceded by one or more qualifiers. There are three different kinds of qualifier: type qualifiers say what kind of thing the id name or number refers to. Possible types are host, net , port and portrange. E.g., `host foo', `net 128.3', `port 20', `portrange 6000-6008'. If there is no type qualif