counter:0:A empty counter:0:B empty counter:1:A first:zero oddlast:one counter:1:B first:zero last:one counter:2:A first:zero odd:one evenlast:two counter:2:B first:zero odd:one last:two counter:3:A first:zero odd:one even:two oddlast:three counter:3:B first:zero odd:one even:two last:three counter:4:A first:zero odd:one even:two odd:three evenlast:four counter:4:B first:zero odd:one even:two odd:three last:four