View Compilation Error

0_0_35339323_31891.cpp: In function 'void Push(int)':
0_0_35339323_31891.cpp:10:5: error: 'a' was not declared in this scope
     a.insert(P);
     ^
0_0_35339323_31891.cpp: In function 'void Pop(int)':
0_0_35339323_31891.cpp:15:34: error: 'a' was not declared in this scope
     multiset<int>::iterator _i = a.upper_bound(P);
                                  ^