Write a function to sort a tree by depth and break ties arbitrarily

[ftyers]

See Algorithm 1 in this paper

In the decoding stage, an unordered tree is given and
the goal is to lift edges that would be non-projective
with  respect  to  the  gold  linear  order.   Similarly  to
how training instances are derived, the decoding al-
gorithm traverses the tree bottom-up and visits every
node once. Ties between nodes at the same depth are
broken in an arbitrary but deterministic way.  When
a node is visited, the classifier is applied and the cor-
responding lifting is executed.  Pseudocode is given
in Algorithm 1.

How this is done is not described, but can be discussed in this issue.