code: def _highest_degree_variable_chooser(problem, variables, domains):
      '''
      Choose the variable that is involved on more constraints.
      '''
      # the variable involved in more constraints
      return sorted(variables, key=lambda v: problem.var_degrees[v], reverse=True)[0f _highest_degree_variable_chooser(problem, variables, domains):
      '''
      Choose the variable that is involved on more constraints.
      '''
      # the variable involved in more constraints
      return sorted(variables, key=lambda v: problem.var_degrees[v], reverse=True)[0]
docs: Choose the variable that is involved on more constraints.