protected void prefixBy(PartitionSpec pSpec) {
      if ( pSpec == null || pSpec.getExpressions() == null) {
        return;
      }
      if ( expressions == null ) {
        expressions = new ArrayList<PTFInvocationSpec.OrderExpression>();
      }
      for(int i = pSpec.getExpressions().size() - 1; i >= 0; i--) {
        expressions.add(0, new OrderExpression(pSpec.getExpressions().get(i)));
      }
    }
Add order expressions from the list of expressions in the format of ASTNode
@param nodes
ql/src/java/org/apache/hadoop/hive/ql/parse/PTFInvocationSpec.java