function value = ch_is_isbn_digit ( c )

%*****************************************************************************80
%
%% ch_is_isbn_digit() returns TRUE if the character C is an ISBN digit.
%
%  Licensing:
%
%    This code is distributed under the MIT license.
%
%  Modified:
%
%    15 September 2015
%
%  Author:
%
%    John Burkardt
%
%  Input:
%
%    character C, a character.
%
%  Output:
%
%    logical VALUE, is TRUE if C is an ISBN digit, FALSE otherwise.
%
  if ( '0' <= c && c <= '9' )
    value = true;
  elseif ( c == 'X' || c == 'x' )
    value = true;
  else
    value = false;
  end

  return
end
