docs: private int calcTZeroDepth(int proposedK)
      {
          int d = -1;
          if ( (proposedK >= 0) && (proposedK < _t0_d.length) )
              d = _t0_d[proposedK];
          if (d < 0)
          {
              String x =
              "K Factor (" + proposedK + ") is invalid.\n" +
              "Valid K factors are: " + kFactorString() + ".";
              throw new IllegalArgumentException(x);
          }
          return d;
      }