SAP PI Mapping - based on a qualifier on source msg

Hi Team,

i have a requirement where the source message is as below. Based on the L1 value , I have to map to different Idoc fields on the target. Can you please let me know how an UDF would look like to achieve this .

<Header>
  <Details>
      <L1> X <L1>
      <L2> D <L1>
<Details>

<Details>
      <L1> Y <L1>
      <L2> Q <L1>
<Details>

<Details>
      <L1> Z <L1>
      <L2> P<L1>
<Details>

....

....

Thanks,

Prem