correct construction, handle successive alleles
correct construction, handle successive alleles

Keep track of the last position in VCF-based construction, and fully connect
nodes to previous nodes in the partial order.

abeconnelly/vg,dnanexus-rnd/vg,ekg/vg,dnanexus-rnd/vg,abeconnelly/vg,ekg/vg,ekg/vg