code: def get_obs_leaves(self):
          
          obs_leaves = []
          for n in self:
              if n.is_leaf:
                  if isinstance(n, ModelNode):
                      l = n.parent
                  else:
                      l = n
              if l not in obs_leaves:
                  obs_leaves.append(l)
          return obs_leaves
docs: Returns the last obs nodes that are leaves