docs: def tree(self):
          """Tree with branch lengths in codon substitutions per site.
  
          The tree is a `Bio.Phylo.BaseTree.Tree` object.
  
          This is the current tree after whatever optimizations have
          been performed so far.
          """
          bs = self.model.branchScale
          for node in self._tree.find_clades():
              if node != self._tree.root:
                  node.branch_length = self.t[self.name_to_nodeindex[node]] * bs
          return self._tree