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Extension argument needs to be able to target any YANG statement, hence requires something like StatementPath in yang-model-api. A statement path is a slightly corrected SchemaPath, which will include information about the namespace in which the lookup needs to take place.

As a first cut, this could look like this:

amend:amend "/d:foo:foo/g:foo:foo/t:foo:foo" {
    units bar;
}

which would be targeting the inner-most statement of

module foo {
    prefix foo;

    container foo {
        grouping foo {
            typedef foo {
               type string;
            }
        }
    }
}

The argument syntax needs to allow for repetitive prefixes to be not need. The prefixes we need are:

• 'c' case: nesting expands implicit cases
• 'd' data: container, list, leaf, leaf-list, choice, rpc, action, notification, anyxml, anydata
• 'e' extension
• 'f' feature
• 'g' grouping
• 'i' identity
• 't' typedef
• 'u' union child type: child offset
• 'x' deviate

The last of which deals with types defined in-line within a union definition, as its member subtypes can be anonymous.

After some prototyping, the structure of argument has changed. The extension will also need to be restricted to prepending extension statements. Furthermore the parser and maven plugin will need to be taught about models which are using this extension, because such models need to be pulled in from the library whenever their target(s) are requested – otherwise incremental builds, such as compile-time of downstreams which reference a model extended in this way would onlyt see the base model, but not the extensions.