Return the namespace ID at the specified index in the prefix stack, where an index of 0 is the oldest ID on the stack. If index is negative, return the ID at position 'numPrefixes() ‐ index'. Thus, an index of ‐1 will return the most recent ID on the stack (i.e., the top of the stack). The behavior is undefined if index > numPrefixes() or index < ‐numPrefixes().