Although they were described by K. Iverson in his 1962 book 1, ordered trees have never been implemented as APL objects. However, APL2 as well as ISO-APL, provide many facilities to handle them. This paper illustrates with simple examples, the duality between the tree-structure and what will be presented as an "object-oriented definition structure". APL functions are proposed to facilitate the conversion of one form into the other one. Trees have great utility in data base processing, as well in science, as in business applications. Interfaces with the "outer world" - e.g. word processors, or LISP and PROLOG can become easy to build with the help of this simple concept of duality.
/lp/association-for-computing-machinery/the-dual-structure-of-ordered-trees-OvBsDHqSCv