code: public GroovyExpression generateAdjacentVerticesExpression(GroovyExpression parent, AtlasEdgeDirection dir) {
          return new FunctionCallExpression(TraversalStepType.FLAT_MAP_TO_ELEMENTS, parent, getGremlinFunctionName(dir));
      }
docs: Generates an expression that gets the vertices adjacent to the vertex in 'parent'
  in the specified direction.
  
  @param parent
  @param dir
  @return