extension PureXML.Schema {
    /// Memoizes the resolved simple type of a complex type's `simpleContent`, keyed by the
    /// complex type's name, during one compile. A `simpleContent` derivation chain (`T0`
    /// extends `T1` extends `T2`, and so on) is flat in the document, so without this each
    /// of the N types re-walks its whole base chain while being compiled, an O(n^3) compile
    /// (a 400-link chain took ~23 s); sharing the resolution makes it linear. A reference
    /// type so it survives the value-type ``XSDContext`` copies (`scoped`/`visiting`); one
    /// instance per compile. An acyclic resolution is intrinsic to the name (a derivation
    /// cycle is rejected elsewhere), so the name is a sufficient key.
    final class SimpleContentMemo {
        var store: [String: PureXML.Schema.SimpleType] = [:]
    }
}