code: public Edge addEdge(final String fromId, final String toId) {
          return getStatements().addEdge(fromId, toId, this);
      }
docs: Convenience method to add and return an Edge between a Node/Subgraph and another Node/Subgraph within the
  immediate Statements in this Graph.
  
  @param fromId The non-empty ID of the Node/Subgraph from which the Edge should originate.
  @param toId   The non-empty ID of the Node/Subgraph to which the Edge should be directed.
  @return The newly created Edge - or {@code null} if the Edge could not be created.