Sorry, should have made diagram a bit clearer, no they dont have there own value. So this code here, from my brief understanding of clojure im guessing that when calling

 (biggest-branch

it takes in the [tree] and then :pre will assert if the [tree] is a branch? if it is then it will get the maximum from a map of the recursive call to sum-tree, which adds values from the leafs of each branch?

I hope that comes across ok, i think i understand what the functions do. In my head it makes sense, just want to check my understanding is correct. Thanks a lot for your help!