sub seq_series {
        my $seqs = seq( @_ );  # seq of seqs (!)
        my $seq;
        seqsub {
            my @val;
            do {
                ($seq) = $seqs->() unless $seq;
                @val = $seq->() if $seq;                
            } while !@val && ($seq = $seqs->());
            @val;
        }
    }

    seq_series( $abcees, $one_two_threes )->enumerate;

    #  0 => a
    #  1 => b
    #  2 => c
    #  3 => 1
    #  4 => 2
    #  5 => 3