  Standpoint logic is a recently proposed formalism in the context of knowledge
integration, which advocates a multi-perspective approach permitting reasoning
with a selection of diverse and possibly conflicting standpoints rather than
forcing their unification. In this paper, we introduce nested sequent calculi
for propositional standpoint logics--proof systems that manipulate trees whose
nodes are multisets of formulae--and show how to automate standpoint reasoning
by means of non-deterministic proof-search algorithms. To obtain worst-case
complexity-optimal proof-search, we introduce a novel technique in the context
of nested sequents, referred to as "coloring," which consists of taking a
formula as input, guessing a certain coloring of its subformulae, and then
running proof-search in a nested sequent calculus on the colored input. Our
technique lets us decide the validity of standpoint formulae in CoNP since
proof-search only produces a partial proof relative to each permitted coloring
of the input. We show how all partial proofs can be fused together to construct
a complete proof when the input is valid, and how certain partial proofs can be
transformed into a counter-model when the input is invalid. These
"certificates" (i.e. proofs and counter-models) serve as explanations of the
(in)validity of the input.
