LAG with WHEN-Clause

Hi,

i'm terribly missing the WHEN-Clause when using the LAG Function as we need to selectively calculate time differences between certain rows in our result sets.

Stream Analytics handles this just fine with the WHEN-Clause:
https://msdn.microsoft.com/en-us/library/azure/dn966240.aspx

Is this on the road map for U-SQL as well? Or do you have any suggestions on achieving a similar result using the existing functions in U-SQL?

Thank you very much!
Kind regards
Markus