Compositional closure of a quadruple (A,B,C,d) means that there are
2 constants and 2 binary operators as follows. The binary
operators
(binding, or tree-forest composition) and
(concatenation, or path-extension composition) are written infix.
The two `star' constructions yield quadruples which are compositionally
closed. The definitions of the concatenation and binding operators are
as follows, by recursion of the first argument: