  In this paper we provide an explicit general construction of higher homotopy
operations in model categories, which include classical examples such as (long)
Toda brackets and (iterated) Massey products, but also cover unpointed
operations not usually considered in this context. We show how such operations,
thought of as obstructions to rectifying a homotopy-commutative diagram, can be
defined in terms of a double induction, yielding intermediate obstructions as
well.
