Meter

// ===============================================================
// As commercial use of electric energy spread in the 1880s, it
// became increasingly important that an electric energy meter,
// similar to the then existing gas meters, was required to
// properly bill customers for the cost of energy, instead of
// billing for a fixed number of lamps per month.
//
//       - WIKIPEDIA (Electricity meter, retrieved on 19 Feb 2017)
// ===============================================================

class Meter {

  private long tally;

  public Meter() {
    tally = 0L;
  }
  public void tick() {
    tally += 1L;
  }
  public long reading() {
    return tally;
  }
}