template<class K, class H, class P, class A, class Predicate>typename unordered_set<K, H, P, A>::size_type
erase_if(unordered_set<K, H, P, A>& c, Predicate pred);
Effects: Equivalent to:
auto original_size = c.size();
for(auto i = c.begin(), last = c.end(); i != last; ){if(pred(*i)){
i = c.erase(i);
}else{++i;
}}return original_size - c.size();