elementAt method
override
Returns the indexth element.
The index must be non-negative and less than length.
Index zero represents the first element (so iterable.elementAt(0) is
equivalent to iterable.first).
May iterate through the elements in iteration order, ignoring the
first index elements and then returning the next.
Some iterables may have a more efficient way to find the element.
Implementation
SpeechGrammar elementAt(int index) => this[index];