*! version 1.0.0   RG   23 December 1993                (STB-25: dm30)
program define compobs
    version 3.1
    local varlist "req ex"
    local if "opt"
    local options "List(string) Number(int 0)"
    parse "`*'"
    parse "`varlist'", parse(" ")
 if `number'==0 {
    local index=index("`if'","=")
    local if2 substr("`if'",`index'+2,.)
    local if4=real(`if2')-1
    tempvar DIFF
    qui gen `DIFF'=0 `if'
    while "`1'"!="" {
         local if3="`if' & `1'~=`1'[_n-1]"
         qui replace `DIFF'=`DIFF' + 1 `if3'
         qui count `if3'
         if _result(1) {
              li `1' `list' `if3' | (_n==`if4' & `1'!=`1'[_n+1])
         }
         mac shift
    }
 }
 else {
    tempvar DIFF
    qui gen `DIFF'=0 `if'
    while "`1'"!="" {
         local index=index("`if'","=")
         local if2 substr("`if'",`index'+2,.)
         local if4=real(`if2')
         local if3="`if' & `1'!=`1'[`number']"
         qui replace `DIFF'=`DIFF' + 1 `if3'
         qui count `if3'
         if _result(1) {
              li `1' `list' `if3' | (_n==`number' & `1'!=`1'[`if4'])
         }
         mac shift
    }
 }
    qui su `DIFF'
    local DIFF2=_result(6)
    noi di ""
    noi di "Number of Differences ="_skip(2) `DIFF2'
end