returns the character representation of a decimal digit.
        </li>
        <li>
          <b>FILE_NAME_INC</b> generates the next filename in a series.
        </li>
        <li>
          <b>GET_UNIT</b> returns a free FORTRAN unit number.
        </li>
        <li>
          <b>HEX_TO_I4</b> converts a hexadecimal string to its integer value.
        </li>
        <li>
          <b>HEX_TO_S</b> converts a hexadecimal string into characters.
        </li>
        <li>
          <b>I4_GCD</b> finds the 