Traverse insertions along - strand if inserted on - strand
Traverse insertions along - strand if inserted on - strand

NB: As a side effect this changes the semantics of constructing a
"reversed" column iterator--it will now go properly go from
right-to-left instead of left-to-right. Shouldn't be a problem, but
needs to be looked into before merging.

glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal,glennhickey/hal