LGTM
https://codereview.appspot.com/547560044/diff/555240043/lily/spring.cc File lily/spring.cc (right): https://codereview.appspot.com/547560044/diff/555240043/lily/spring.cc#newcode119 lily/spring.cc:119: avg_stretch /= static_cast<Real> (springs.size ()); This is not an issue with this change, but should this function do something other than divide by zero when springs is empty? https://codereview.appspot.com/547560044/
