code: public FaunusPipeline linkIn(final String label, final String step) {
          return this.link(IN, label, step, null);
      }
docs: Have the elements for the named step previous project an edge to the current vertex with provided label.
  
  @param step  the name of the step where the source vertices were
  @param label the label of the edge to project
  @return the extended FaunusPipeline