sclass Thoughts {
  new LinkedHashSet<Lisp> statements;
  new MultiMap<S, Lisp> statementsByHead;
  
  bool add(Lisp statement) {
    if (statement == null) ret;
    bool isNew = statements.add(statement);
    if (isNew)
      statementsByHead.put(statement.head, statement);
    ret isNew;
  }
}