Iterator linearProduce(Concept c) {
  if (c.closed)
    return new ListIterator(c.stringExamples);
  S[] s = c.matchDescription("concatenation of *");
  if (s != null) {
    Concept element = singular(s[0]);
    return produceConcatenation(element);
  }
}

Iterator produceConcatenation(Concept c) {
  return new Iterator() {
    static final int n = 2;
    boolean hasNext() { return i < n; }
    (""); todo
  }
}