foo.cc: In member function 'SeqList SeqList::operator+(const SeqList&)':
foo.cc:67:9: error: 'i' was not declared in this scope
   67 |   while(i<n) ss.push_back(a[i]), i++;
      |         ^
foo.cc:68:9: error: 'j' was not declared in this scope
   68 |   while(j<s.n) ss.push_back(s.a[j]), j++;
      |         ^

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