code: List<Coordinate> allMatches() {
          List<Coordinate> pairs = new ArrayList<>();
          int rows = rowList().size();
          int cols = columnList().size();
          for (int i = 0; i < rows; i++) {
              for (int j = 0; j < cols; j++) {
                  VariantGraph.Vertex vertex = vertexAt(i, j);
                  if (vertex != null) pairs.add(new Coordinate(i, j, new Match(vertex, null)));
              }
          }
          return pairs;
      }
docs: TODO: try remove the call to rowList / columnList