function [tvals, yvals]=Unstab(fname,h,tmax)

// This function is for instruction only. The method is unstable.

n=tmax/h;

   tcur = h; 
   yold = 1; ycur = exp(h);
   fold = fname(0, yold); fcur = fname(tcur, ycur);

    for j=2:n
      ynew = -4*ycur + 5*yold + h*(4*fcur + 2*fold);
      tcur = tcur+h;
      tvals(j) = tcur;
      yvals(j) = ynew;
      fnew = fname(tcur,ynew);
      yold = ycur; ycur = ynew;
      fold = fcur; fcur = fnew;
    end;
    plot2d(tvals, yvals)