variable var in constraint const ""return {(nvarnconstfor nconst in self csp var_to_const[varif nconst !const for nvar in nconst scope if nvar !varthe following selects an arc any element of to do can be selected the selected element needs to be removed from to do the default implementation just selects which ever element pop method for sets returns user interface could allow the user to select an arc alternatively more sophisticated selection could be employed (or just stack or queuecspconsistency py -(continued def select_arc(selfto_do)"""selects the arc to be taken from to_do to_do is set of arcswhere an arc is (variable,constraintpair the element selected must be removed from to_do ""return to_do pop(the value of new_domain is the subset of the domain of var that is consistent with the assignment to the other variables it might be easier to understand the following codewhich treats unary (with no other variables in the constraintand binary (with one other variables in the constraintconstraints as special cases (this can replace the assignment to new_domain in the above code)if len(other_vars)== unary constraint new_domain {val for val in domains[varif const holds({var:val})elif len(other_vars)== binary constraint other other_vars[ new_domain {val for val in domains[varif any(const holds({varval,other:other_val}for other_val in domains[other])elsegeneral case new_domain {val for val in domains[varif self any_holds(domainsconst{varval}other_vars)any holds is recursive function that tries to finds an assignment of values to the other variables (other varsthat satisfies constraint const given the assignment in env the integer variable ind specifies which index to other vars needs to be checked next as soon as one assignment returns truethe algorithm returns true note that it has side effects with respect to envit changes the values of the variables in other vars it should only be called when the side effects have no ill effects cspconsistency py -(continuedversion june