+Real
+Line_details::spring_length (Line_details const &next_line) const
+{
+ // space_ measures the spring which goes from the bottom refpoint
+ // of this to the top refpoint of next_line. We want to return
+ // the stretchable space between the bottom of this's extent to
+ // the top of next_line's extent.
+ Real refpoint_dist = tallness_ + refpoint_extent_[DOWN] - next_line.refpoint_extent_[UP];
+ Real space = next_line.title_ ? title_space_ : space_;
+ return max (0.0, space - refpoint_dist);
+}
+