ml#ZOLTAN_PRE_MIGRATE_PP_FN"><b>ZOLTAN_PRE_MIGRATE_PP_FN</b></a>,
<a href="ug_query_mig.html#ZOLTAN_MID_MIGRATE_PP_FN"><b>ZOLTAN_MID_MIGRATE_PP_FN</b></a>, and 
<a href="ug_query_mig.html#ZOLTAN_POST_MIGRATE_PP_FN"><b>ZOLTAN_POST_MIGRATE_PP_FN</b></a>.
If all processors pass NULL arguments for the export arrays, the value of <i>num_export</i> should be -1.
In this case, <b>Zoltan_Migrate</b> computes the export lists based on the
import lists.
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