      program prog081
      .....
      ga = gam*a*a
      .....
c IF GOTO LOOP Start
 10   continue
         call onestep(x,y,vx,vy,t,dt,ga)
         .....
         write(*,....) x,y,t
      if( y .ge. 0.0d0 ) goto 10
c IF GOTO LOOP End
      .....
      end
c **********************************************************************
      subroutine onestep(x,y,vx,vy,t,dt,ga)
c **********************************************************************
      .....
      call force(vx, vy, fx, fy, ga)
      .....
      call force(vx1,vy1,fx1,fy1,ga)
      .....
      dx =  .....
      .....
c Real Propagation in Improved Euler method
      x = x + dx
      .....
      t = t + dt
      end
c **********************************************************************
      subroutine force(vx,vy,fx,fy,ga)
c **********************************************************************
      .....
      fx = .....
      .....
      end
