final char openBracket = possibleBracketsInStructure[b][0];
final char closeBracket = possibleBracketsInStructure[b][1];
stack.clear();
for (int i = 0; i < secondary.length(); i++) {
if (secondary.charAt(i) == openBracket) {
stack.push(i);
} else if (secondary.charAt(i) == closeBracket) {
final int start = stack.pop();
ans.add(new IntPair(start, i));
}
}