* @param a {@link Document}
* @param b {@link Document}
* @return merged {@link ProtoNetwork}
*/
private ProtoNetwork merge(Document a, Document b) {
ExpansionService es = new ExpansionServiceImpl();
ProtoNetworkBuilder builder = new ProtoNetworkBuilder(a);
ProtoNetwork networkA = builder.buildProtoNetwork();
es.expandTerms(a, networkA);
builder = new ProtoNetworkBuilder(b);
ProtoNetwork n2 = builder.buildProtoNetwork();
es.expandTerms(b, n2);
ProtoNetworkMerger merger = new ProtoNetworkMerger();
merger.merge(networkA, n2);
return networkA;
}