title: OrientDB: Find all vertices that have no direct neighbour vertex of a given class
body: <p>Using OrientDB's query language, how can find all vertices in cluster <code>a</code> that have no outgoing edge ending in a vertex of class <code>b</code> (i.e. no direct neighbour vertex of class <code>b</code>)?
  It does not matter if they have other outgoing edges.</p>