Conex_utils.TreeSourceTree is a simple tree datatype, edge is a string, values are 'a lists.
The main tree type
equal eq a b compares a with b, using eq to compare values.
fold f acc t folds f over t, using the accumulator acc.
pp pp_e ppf t pretty prints the tree t using pp_e for printing values.
lookup_prefix path t finds the closest non-empty 'a on path.