Appeared in J. Computational and Applied Math 32 (1990), 321-368.
Special Functions are very old, since the trig. functions are really special. There were quite a few succesful approaches to put Special functions in a General context, be it combinatorial (Foata), Algebraic (Wigner, Gelfand), or Analytical (Truesdal). But, I believe that the approach described in this article is, in some sense, the best, since it is Algorithmic, i.e. it makes any Identity (of the right form) routine.
Doron Zeilberger's List of Papers