Using the mutable tree and its mergeInto function introduced ingist:6074217

gistfile1.fs

  let tree = ref Tree.Empty
  let branch1 = ["a";"b";"c"]
  let branch2 = ["a";"b";"d"]
  let branch3 = ["a";"f"]
  
  do mergeInto tree branch1
  do mergeInto tree branch2
  do mergeInto tree branch3
  //the following tree is created
  //        a
  //      /   \
  //    f       b
  //          /   \
  //        d       c