+ The Span_bar_stub_engraver creates SpanBarStub grobs in the contexts
+ that a grouping context contains. For example, if a PianoStaff contains
+ two Staffs, a Dynamics, and a Lyrics, SpanBarStubs will be created in
+ all contexts that do not have bar lines (Dynamics and Lyrics).
+
+ We only want to create these SpanBarStubs in contexts that the SpanBar
+ traverses. However, Contexts do not contain layout information and it
+ is thus difficult to know if they will eventually be above or below other
+ Contexts. To determine this we use the VerticalAxisGroup created in the
+ Context. We relate VerticalAxisGroups to Contexts in the variable
+ axis_groups_ and weed out unused contexts after each translation timestep.
+
+ Note that SpanBarStubs exist for pure height calculations ONLY.