Synchronizing flow objects
Constrain relative position of flow objects in different streams.
For example, constrain position of footnotes/floating
figures/sidenodes relative to reference.
For example, constrain parallel translations to start each paragraph
at same vertical point.
When constructing flow object tree, can specify that flow objects
are part of a synchronization set
Each synchronization set must have synchronizing flow object that
contains either directly or indirectly all members of synchronization
set. Each stream of synchronizing flow object can contain at most
one member of synchronization set.
For synchronization sets with two members, can constrain placement of
members so that number of pages/spread/columns between them is between
For synchronization sets with arbitrarily many members, can constrain
members of synchronization set to have same position in column-set