program reflective_walls
      write(*,*)'Vx?'
      read(*,*)vx
      write(*,*)'Vy?'
      read(*,*)vy

      i_dummy=pgopen('/xwin')
      xmin=-10
      xmax=10
      ymin=-10
      ymax=10
      r=0.3
      wdth=+(1+0.1)*r
      call pgenv(xmin-wdth,xmax+wdth,ymin-wdth,ymax+wdth,1,-1)
      call pgsfs(2)

      x=0
      y=0
      xold=x
      yold=y
      dt=0.00001
      do i=1,10000000

        if(x>xmax) vx=(-1)*vx
        if(x<xmin) vx=(-1)*vx
        if(y>ymax) vy=(-1)*vy
        if(y<ymin) vy=(-1)*vy

        x=x+vx*dt
        y=y+vy*dt
        call pgbbuf
        call pgsci(0)
        call pgcirc(xold,yold,r)
        call pgsci(1)
        call pgcirc(x,y,r)
        call pgebuf
        xold=x
        yold=y
      end do
      call pgclos
      end