docs: private static int EquivalentYear(int year)
      {
          int day = (int) DayFromYear(year) + 4;
          day = day % 7;
          if (day < 0)
              day += 7;
          // Years and leap years on which Jan 1 is a Sunday, Monday, etc.
          if (IsLeapYear(year)) {
              switch (day) {
                  case 0: return 1984;
                  case 1: return 1996;
                  case 2: return 1980;
                  case 3: return 1992;
                  case 4: return 1976;
                  case 5: return 1988;
                  case 6: return 1972;
              }
          } else {
              switch (day) {
                  case 0: return 1978;
                  case 1: return 1973;
                  case 2: return 1985;
                  case 3: return 1986;
                  case 4: return 1981;
                  case 5: return 1971;
                  case 6: return 1977;
              }
          }
          // Unreachable
          throw Kit.codeBug();
      }