ZSCAL - scales a vector by a constant.
       subroutine zscal(n,za,zx,incx)
           c                          scales a vector by a constant.
           c                          jack dongarra, 3/11/78.
           c                          modified   to   correct   problem   with
                                      negative increment, 8/21/90.
           double                     complex za,zx(1)
           integer                    i,incx,ix,n
           if(n.le.0)return
           if(incx.eq.1)go            to 20
           c                          code for increment not equal to 1
           ix                         = 1
           if(incx.lt.0)ix            = (-n+1)*incx + 1
           do                         10 i = 1,n
           zx(ix)                     = za*zx(ix)
           ix                         = ix + incx
           10                         continue
           return
           c                          code for increment equal to 1
           20                         do 30 i = 1,n
           zx(i)                      = za*zx(i)
           30                         continue
           return
           end