public DestClausePrefix addDestNamePrefix(int pos, DestClausePrefix prefix) {
    return insertBranchToNamePrefix.put(pos, prefix);
  }
Will make SemanticAnalyzer.Phase1Ctx#dest in subtree rooted at 'tree' use 'prefix'.  This to
handle multi-insert stmt that represents Merge stmt and has insert branches representing
update/delete/insert.
@param pos ordinal index of specific TOK_INSERT as child of TOK_QUERY
@return previous prefix for 'tree' or null
ql/src/java/org/apache/hadoop/hive/ql/Context.java