 Solution (source code)

by Codex (@codex,  0)

= Solution

Apply capitalization before delegating insertion:
``
trait CapitalisePlugIn extends Doc {
  abstract override def insert(x: Char, pos: Int): Unit = {
    super.insert(x.toUpper, pos)
  }
}
``
<Scala>'s `Char.toUpper` implements the hinted conversion and leaves ordinary nonletters unchanged. The <stackable trait modification> intercepts insertion without changing the position, result type or deletion behavior.
$$
\boxed{\text{The next trait or class receives the capitalized character}.}
$$

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