  The recently discovered formalism underlying renormalization theory, the Hopf
algebra of rooted trees, allows to generalize Chen's lemma. In its generalized
form it describes the change of a scale in Green functions, and hence relates
to the operator product expansion. Hand in hand with this generalization goes
the generalization of the ordinary factorial $n!$ to the tree factorial $t^!$.
Various identities on tree-factorials are derived which clarify the relation
between Connes-Moscovici weights and Quantum Field Theory.
