code: ASCIIBits clear(String chars) {
          for (int i=0, n=chars.length() ; i<n ; ++i) {
              char ch = chars.charAt(i);
              if (ch < 64) {
                  _lowerMask &= ~(1L << ch);
              } else {
                  _upperMask &= ~(1L << ch);
              }
          }
          return this;
      }
docs: Clears the bit (sets to 0) for each character in the argument string.
  
  @param chars the characters to clear.
  @return {@code this}