complex(8) function zacos (z)
      implicit none
c april 1977 version.  w. fullerton, c3, los alamos scientific lab.
      complex(8) z, zasin
      real(8) pi2
      external zasin
      data pi2 /1.5707963267 9489661923d0/
c
      zacos = pi2 - zasin (z)
c
      return
      end