The skos:inScheme property is no longer a sub-property of rdfs:isDefinedBy, because the latter in fact expresses more precise semantics. The latter should be used to express a relationship between a concept and its original, defining, concept scheme (if one exists). Use the skos:inScheme property expresses a relationship between a concept and any concept scheme in which it participates, which may or may not be the original, defining concept scheme.
(This change to better handle inclusion of concepts in multiple schemes).