question about transposing an interval of a 4th

[chip]

I am inputing a piece in concert C, the use \transpose to output a part 
for a Bb Trumpet. Now I want to add second part that will be a fourth 
lower. Can Lily do this or do I have to figure out all the notes a 
fourth lower and enter them manually? I'd rather just copy/paste the 
first part into the second part and let Lily do the transposing of a 
fourth interval.
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Chip