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