program spiral
      write(*,*)'input a of r=a* theta:'
      read(*,*)a
      write(*,*)'up to what angle theta?'
      read(*,*)theta_max
      d_theta=0.01
      n=int(theta_max/d_theta)
      if(pgopen('/xwin')<=0)stop
      call pgenv(-10.0,10.0,-10.0,10.0,1,0)
      do i=1,n
        theta=0 + i*d_theta
        r = a*theta
        x=r*cos(theta)
        y=r*sin(theta)
        call pgpt(1,x,y,20)
      end do
      call pgclos
      end