Composition of ABNF grammars.
Certain ABNF grammars are defined modularly. A ``module'' may not be closed, but when it is combined with other modules, the resulting grammar may be closed.
As another example, the HTTP grammar specified in RFC 7230 includes rules defined by prose value notations that refer to the URI grammar specified in RFC 3968. The intended way to compose the two grammars is to replace the prose HTTP rules with the corresponding URI rules.
Here we define an operation to accomplish the kind of composition exemplified above. The operation ``plugs'' a rule list into another rule list, e.g. it plugs the core rules into the concrete syntax rules, and it plugs the URI rules into the HTTP rules.