code: def getArcs(domains, constraints):
      
      arcs = {}
      for x in constraints:
          constraint, variables = x
          if len(variables) == 2:
              variable1, variable2 = variables
              arcs.setdefault(variable1, {}).setdefault(variable2, []).append(x)
              arcs.setdefault(variable2, {}).setdefault(variable1, []).append(x)
      return arcs
docs: Return a dictionary mapping pairs (arcs) of constrained variables
  
      @attention: Currently unused.