complex(8) function zatan2 (csn, ccs)
        implicit none
  c april 1977 version.  w. fullerton, c3, los alamos scientific lab.
        complex(8) csn, ccs, zatan
        real(8) pi
        external zatan
        data pi / 3.1415926535 8979323846d0 /
  c
        if (abs(ccs).eq.0.) go to 10
  c
        zatan2 = zatan (csn/ccs)
        if (real(ccs).lt.0.) zatan2 = zatan2 + pi
        if (real(zatan2).gt.pi) zatan2 = zatan2 - 2.0*pi
        return
  c
   10   if (abs(csn).eq.0.) call seteru (
       1  34hzatan2  called with both args zero, 34, 1, 2)
  c
        zatan2 = dcmplx (sign(0.5*pi,real(csn)), 0.0)
  c
        return
        end