private static int adjustPageUsefulness(@NotNull PageImpl page) {
    int usageCount = page.usageCount();
    if (usageCount > 0) {
      return page.addTokensOfUsefulness(usageCount * TOKENS_PER_USE);
    }
    else {//exponential decay of usefulness:
      return page.decayTokensOfUsefulness(TOKENS_PER_USE - 1, TOKENS_PER_USE);
    }
  }
private static int adjustPageUsefulness(@NotNull PageImpl page)