*! kornbrot -- Kornbrot's rank difference test                  (STB-29: snp9)
program define kornbrot
    version 4.0
    local varlist "req ex max(1)"
    local exp "req noprefix"
    local if "opt"
    local in "opt"
    parse "`*'"
    tempvar score rank ranka rankb touse id

    if "`exp'" ~= "" {
         unabbrev `exp'
    }

    preserve

    quietly {
         mark `touse' `if' `in'
         markout `touse' `varlist' `exp'
         drop if `touse'==0
    }

    quietly {
         su `varlist'
         local en _result(1)
         gen `id'=_n
         expand=2
         gen `score'=`varlist'
         replace `score'=`exp' if _n>`en'
         egen `rank'=rank(`score')
         replace `rank'=((2*`en')+1) - `rank'
         gen `ranka'=`rank' if `varlist'==`score'
         gen `rankb'=`rank' if `exp'==`score'
         sort `id' `ranka'
         replace `rankb'=`rankb'[_n+1] if `rankb'==.
         drop if `ranka'==.
    }
    signrank `ranka'=`rankb'
end