  Structural resolution (or S-resolution) is a newly proposed alternative to
SLD-resolution that allows a systematic separation of derivations into
term-matching and unification steps. Productive logic programs are those for
which term-matching reduction on any query must terminate. For productive
programs with coinductive meaning, finite term-rewriting reductions can be seen
as measures of observation in an infinite derivation. Ability of handling
corecursion in a productive way is an attractive computational feature of
S-resolution.
  In this paper, we make first steps towards a better conceptual understanding
of operational properties of S-resolution as compared to SLD-resolution. To
this aim, we propose a type system for the analysis of both SLD-resolution and
S-resolution.
  We formulate S-resolution and SLD-resolution as reduction systems, and show
their soundness relative to the type system. One of the central methods of this
paper is realizability transformation, which makes logic programs productive
and non-overlapping. We show that S-resolution and SLD-resolution are only
equivalent for programs with these two properties.
