%! no DSC

/on {
    dup length dict /k exch def
    {
        dup
        k exch known {
            k exch undef
        } {
            k exch 1 put
        } ifelse
    } forall
    k {
        pop
        /out exch def
        exit
    } forall
    out
} def

[ 2 5 4 4 5 5 2 ] on 5 eq { (Pass) } { (Fail) } ifelse =

[ 1 2 3 4 3 2 1 4 4 ] on 4 eq { (Pass) } { (Fail) } ifelse =