open class a {
  Metadata: Mapping
}

aa = new a {
  Metadata {
    ["Foo"] {
        ["Key1"] = "bar"
    }
  }
}

bb = (aa) {
  Metadata {
    ["Foo"] {
        ["Key1"] = "baz"
        ["Key2"] = "baz2"
    }
  }
}