For example the only difference between the two leftmost trees in Figure 2 is a modifier, but δ plain gives them distance 4 and δ dif f 0.
δ dif f might underestimate some distances as well; for exam-ple the leftmost and rightmost trees also have distance zero using δ dif f , despite our syntactic intuition that the difference between a transitive and an intransitive should be taken account of.