program particle_in_box_animate
      integer pgopen
      write(*,*)'What is initial vx?'
      read(*,*) vx
      write(*,*)'What is initial vy?'
      read(*,*) vy

      x_range = 10.0
      y_range = 10.0
      x = 5.0
      y = 5.0

      if( pgopen('/xwin') <= 0 ) stop
c     call pgenv(0.0,x_range,0.0,y_range,0,-1)
      call pgenv(-0.15,x_range+0.15,-0.15,y_range+0.15,0,-1)
      call pgsfs(2)

      dt = 0.00001

      do i =1, 1000000
        if(x>x_range) vx = (-1)*vx
        if(x<0.0) vx = (-1)*vx
        if(y>y_range) vy = (-1)*vy
        if(y<0.0) vy = (-1)*vy
        x = x + vx * dt
        y = y + vy * dt

        call pgbbuf
        call pgsci(0)
        call pgcirc(x_old,y_old,0.1)
        call pgsci(1)
        call pgcirc(x,y,0.1)
        call pgebuf

        x_old = x
        y_old = y
      enddo
      call pgclos
      end