program square_rot_sub
      real x(5),y(5),xrot(5),yrot(5)
      x(1)=-1
      y(1)=1
      x(2)=1
      y(2)=1
      x(3)=1
      y(3)=-1
      x(4)=-1
      y(4)=-1
      x(5)=x(1)
      y(5)=y(1)

      i_dummy=pgopen('/xwin')
      call pgenv(-5.0,5.0,-5.0,5.0,1,0)

      dtheta=3.14/18000

c 1000 do i=1,5
c       xrot(i)=x(i)*cos(dtheta)-y(i)*sin(dtheta)
c       yrot(i)=x(i)*sin(dtheta)+y(i)*cos(dtheta)
c     end do

 1000 call rot(5,x,y,xrot,yrot,dtheta)

      call pgbbuf
       call pgsci(0)
       call pgline(5,x,y)
       call pgsci(1)
       call pgline(5,xrot,yrot)
      call pgebuf

      do i=1,5
        x(i)=xrot(i)
        y(i)=yrot(i)
      end do

      goto 1000

      end

      subroutine rot(n,x,y,xrot,yrot,dtheta)
      real x(100),y(100),xrot(100),yrot(100)
      do i=1,n
        xrot(i)=x(i)*cos(dtheta)-y(i)*sin(dtheta)
        yrot(i)=x(i)*sin(dtheta)+y(i)*cos(dtheta)
      end do
      end