. In the design of a functional library in the area of data-mining several algorithmic patterns have been identified which call for generic programming. Some of these have to do with flattening functions which arise in a particular group of hierarchical systems. In this paper we describe our efforts to make such functionalities generic. We start by a generic inductive construction of the intended class of hierarchical types. We conclude by relating the structure of the relevant base-functors with the algebraic structure which is required by the generic flattening functionality, in particular concerning its "deforestation" towards a linearly complex implementation. The instances we provide as examples include the widely known bill of materials "explode" operation. 1 Introduction The definition of a function f : B \Gamma! A (1) can be regarded as a kind of "contract": function f is committed to produce an A-value provided it is supplied with a B-value. Such "functional contracts" ca...

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