That's a hard one to get right; for a long time, C++ compilers would tokenize 'vector<vector<int>>' incorrectly and throw a syntax error -- the final two angle

That's a hard one to get right; for a long time, C++ compilers would tokenize 'vector<vector<int>>' incorrectly and throw a syntax error -- the final two angle brackets had to be separated by a space for the code to compile.