if Arg in <X,Y,Z>
(...)
end if    

macro ifin?! candidate, matches
        local list, result, tester

        iterate item, matches
                define list.item
        end iterate

        calminstruction (out) tester in
                local state
                transform in, list
                jyes true
                compute state, 0
                jump done
            true:
                compute state, 1
            done:
                publish out, state
        end calminstruction

        result tester candidate
        if result
end macro     

macro try Arg
        ifin Arg, <X,Y,Z>
                display 'Yes'
        else
                display 'No'
        end if
end macro

try Z