age have
 generic type parameters that correspond to the field value type(s), plus a final generic type parameter
 corresponding to the "target type". For example, an index on field <code>int </code><code>getAccountNumber()</code> of type
 <code>User</code> will be represented by a <a href="Index1.html" title="interface in io.permazen.index"><code>Index1</code></a><code>&lt;Integer, User&gt;</code>, and may be viewed
 either as a <a href="https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/uti