program rot_square
      real x(5),y(5),xold(5),yold(5)
      if(pgopen('/xwin')<=0)stop
      call pgenv(-2.0,2.0,-2.0,2.0,1,0)
C Initial vertex values
      xold(1)=1.0
      yold(1)=1.0
      xold(2)=1.0
      yold(2)=-1.0
      xold(3)=-1.0
      yold(3)=-1.0
      xold(4)=-1.0
      yold(4)=1.0
      xold(5)=1.0
      yold(5)=1.0

      dtheta=0.0001
      do i=1,100000
        do j=1,5
          x(j)=xold(j)*cos(dtheta)+yold(j)*sin(dtheta)
          y(j)=(-1)*xold(j)*sin(dtheta)+yold(j)*cos(dtheta)
        end do
        call pgbbuf
        call pgsci(0)
        call pgline(5,xold,yold)
        call pgsci(1) 
        call pgline(5,x,y)
        call pgebuf
        do k=1,5
          xold(k)=x(k)
          yold(k)=y(k)
        end do
      end do
      call pgclos
      end