code: def mac(csp, var, value, assignment, removals):
      "Maintain arc consistency."
      return AC3(csp, [(X, var) for X in csp.neighbors[var]], removalsf mac(csp, var, value, assignment, removals):
      "Maintain arc consistency."
      return AC3(csp, [(X, var) for X in csp.neighbors[var]], removals)
docs: Maintain arc consistency.