the chain to prepend to, which must already have at least one bone
the direction the new bone points, from its own start toward the existing chain. Assumed to be unit length
the length of the new bone
the joint for the junction this creates, or a fresh unconstrained one if omitted
the prepended bone
Prepends a bone at the base end, extending the chain backward.
This is what a follower grows with: driven by forward alone the base end is the tail.
jointbecomes the joint of the bone that was previously first, because that is the junction the new bone creates. The new first bone's own joint is unused, as always.