expo( x )

   function s = expo( x )

    t = 1.0; n = 1.0; s = 0.0;
    while 1
      if ( abs(t) <=  1.0e-14 )
        break;
      end; 	
      s = s + t;
      t = t * (x/n);
      n = n + 1.0;
    end