program square_rot
      real x(5),y(5),x_rot(5),y_rot(5),xold(5),yold(5)
      x(1)= 1.0
      y(1)= 1.0
      x(2)=-1.0
      y(2)= 1.0
      x(3)=-1.0
      y(3)=-1.0
      x(4)= 1.0
      y(4)=-1.0
      x(5)= x(1) 
      y(5)= y(1)

      theta=0.0
      d_theta = 3.14/18000

      do i=1,5
        xold(i)=x(i)
        yold(i)=y(i)
      end do

      i_dummy=pgopen('/xwin')
      call pgenv(-4.0,4.0,-4.0,4.0,1,0)

 1000 theta = theta + d_theta

      do i=1,5
        x_rot(i) =      x(i)*cos(theta) + y(i)*sin(theta)
        y_rot(i) = (-1)*x(i)*sin(theta) + y(i)*cos(theta)
      end do

      call pgbbuf
      call pgsci(0)
      call pgline(5,xold,yold)
      call pgsci(1)
      call pgline(5,x_rot,y_rot)
      call pgebuf

      do i=1,5
        xold(i)=x_rot(i)
        yold(i)=y_rot(i)
      end do

      goto 1000

      end