sclass StrictHashMap<A, B> extends HashMap<A, B> {
  bool detailedExceptions;
  
  public B put(A key, B value) {
    B v = get(key);
    if (v != null && neq(v, value))
      if (detailedExceptions)
        fail("Strict map conflict: " + structure(key) + ", " + structure(get(key)) + ", " + structure(value));
      else
        fail("Trying to change strict map.");
    ret super.put(key, value);
  }
}

Began life as a copy of #1003482