old_contents: from .core import (CallbackBase, CallbackCounter, print_metadata, collector,
                     LiveMesh, LivePlot, LiveRaster, LiveTable, CollectThenCompute,
                     _get_obj_fields)
  
new_contents: from .core import (CallbackBase, CallbackCounter, print_metadata, collector,
                     LiveMesh, LivePlot, LiveRaster, LiveTable, CollectThenCompute,
                     LiveSpecFile, _get_obj_fields)
  
subject: Add LiveSpecFile to callbacks API.