code: def min_edge_cover(G, matching_algorithm=None):
      """Returns a set of edges which constitutes
      the minimum edge cover of the graph.
  
      The smallest edge cover can be found in polynomial time by finding
      a maximum matching and extending it greedily so that all nodes
      are covered.
  
      Parameters
      ----------
      G : NetworkX graph
          An undirected bipartite graph.
  
      matching_algorithm : function
          A function that returns a maximum cardinality matching in a
          given bipartite graph. The function must take one input, the
          graph ``G``, and return a dictionary mapping each node to its
          mate. If not specified,
          :func:`~networkx.algorithms.bipartite.matching.hopcroft_karp_matching`
          will be used. Other possibilities include
          :func:`~networkx.algorithms.bipartite.matching.eppstein_matching`,
  
      Returns
      -------
      set
          A set of the edges in a minimum edge cover of the graph, given as
          pairs of nodes. It contains both the edges `(u, v)` and `(v, u)`
          for given nodes `u` and `v` among the edges of minimum edge cover.
  
      Notes
      -----
      An edge cover of a graph is a set of edges such that every node of
      the graph is incident to at least one edge of the set.
      A minimum edge cover is an edge covering of smallest cardinality.
  
      Due to its implementation, the worst-case running time of this algorithm
      is bounded by the worst-case running time of the function
      ``matching_algorithm``.
      """
      if G.order() == 0:  # Special case for the empty graph
          return set()
      if matching_algorithm is None:
          matching_algorithm = hopcroft_karp_matching
      return _min_edge_cover(G, matching_algorithm=matching_algorithm)
  
imports: import networkx
  from networkx.algorithms.bipartite.matching import hopcroft_karp_matching
  from networkx.algorithms.covering import min_edge_cover
  from networkx.utils import not_implemented_for
label_desc: Use image node_id 1 for calling a global function with example usage: min_edge_cover(G, matching_algorithm) and returns: _min_edge_cover, set