docs: def __is_subgraph_planar(graph):
      """Internal function to determine if a subgraph is planar."""
      # --First pass: Determine edge and vertex counts validate Euler's Formula
      num_nodes = graph.num_nodes()
      num_edges = graph.num_edges()
  
      # --We can guarantee that if there are 4 or less nodes, then the graph is planar
      # --A 4-node simple graph has a maximum of 6 possible edges (K4); this will always satisfy Euler's Formula:
      # -- 6 <= 3(4 - 2)
      if num_nodes < 5:
          return True
  
      if num_edges > 3*(num_nodes - 2):
          return False
  
      # --At this point, we have no choice but to run the calculation the hard way
      return kocay_planarity_test(graph)