A regular term (Definition C.5.3 of Transducers) is an expression built by applying combinators to atomic terms. The atomic terms are the identity $A \to A$, the projections $A \times B \to A$ and $A \times B \to B$, the co-projections $A \to A + B$ and $B \to A + B$, distributivity $A \times (B + C) \to (A \times B) + (A \times C)$, the list constructor $\mathbb{1} + A \times A^* \to A^*$ and deconstructor $A^* \to \mathbb{1} + A \times A^*$, reverse $A^* \to A^*$, concatenation $A^{**} \to A^*$, split $(A + B)^* \to A^* \times (B \times A^*)^*$, and group prefix multiplication $G^* \to G^*$ for a group $G$ whose underlying set is a finite type. The combinators are composition, pairing $(f, g) : A \to B \times C$, co-pairing $\mathsf{either}\ f\ g : A + B \to C$, and map $f^* : A^* \to B^*$. Every regular term defines a type-to-type function; Theorem C.5.4 relates the functions so defined to the regular functions under string representation.