Musashi | Motorola 680x0 emulator written in C

The example you have posted demonstrate one of the corner cases of the SPARQL specification, which combines multiple related topics and are highly ambiguous in my opinion. The details below explain what are the taken assumptions and design decisions in the GraphDB engine. Please note that this might be different from the way other implementations read the following specification lines:

Interplay of SERVICE and VALUES

The SPARQL Federation 1.1 has a non-normative section describing what should be the behavior in this case:

Implementers of SPARQL 1.1 Federated Query may use the VALUES clause to constrain the results received from a remote endpoint based on solution bindings from evaluating other parts of the query.

GraphDB's query optimizer cannot retrieve any statistics from the remote SPARQL endpoint, so it takes the approach to throw naively the query to the remote SERVICE and join locally the results. Thus, the query optimization task is in the hands of the user who knows the schema in the two repositories by rearranging the query in a procedural way (see below).

Federated queries are sub-queries

Every remote query is treated as a sub-query and sent as it is to the external endpoint. Here is the equivalent syntax: