sclass ThoughtSpace {
  new L<LispStatement> statements;
  new HashSet<Lisp> statementsIndex;
  new LinkedHashMap<S, SimpleConcept> concepts;
  
  void addStatement(Lisp l) {
    if (!containsStatement(l)) {
      statements.add(new LispStatement(l));
      statementsIndex.add(l);
    }
  }
  
  bool containsStatement(Lisp l) {
    ret statementsIndex.contains(l);
  }
}