public static ExprNodeDesc backtrack(ExprNodeDesc source, Operator<?> current,
      Operator<?> terminal, boolean foldExpr) throws SemanticException {
    return backtrack(source, current, terminal, foldExpr, false);
  }
Convert expressions in current operator to those in terminal operator, which
is an ancestor of current or null (back to top operator).
Possibly contain null values for non-traceable exprs
ql/src/java/org/apache/hadoop/hive/ql/plan/ExprNodeDescUtils.java