I’ve come to believe that I’m stumped in querying the existence of (s,t)-paths with SPARQL 1.0 or the k nearest neighbors with SPARQL 1.1 because of limitations of the query language, not limitations in my query-writing ability. But how to prove it? Or even provide some evidence? Tell me on cstheory.stackexachange.
Maybe this paper is relevant: Semantics and Complexity of SPARQL
Healthy Algorithms 