function ind2key( ind ) {
			if ( this !== tuple ) { // eslint-disable-line no-invalid-this
				throw new TypeError( 'invalid invocation. `this` is not host tuple.' );
			}
			if ( !isInteger( ind ) ) {
				throw new TypeError( format( 'invalid argument. Must provide an integer. Value: `%s`.', ind ) );
			}
			if ( ind < 0 ) {
				ind = nfields + ind;
			}
			if ( ind < 0 || ind >= nfields ) {
				return;
			}
			return fields[ indices[ ind ] ];
		}
Converts a tuple index to a field name.

## Notes

-   If provided an out-of-bounds index, the function returns `undefined`.
-   If provided a negative tuple index, the function resolves the index relative to the last tuple element.

@private
@memberof tuple
@param {integer} ind - tuple index
@throws {TypeError} `this` must be the host tuple
@throws {TypeError} must provide an integer
@returns {(string|void)} field name or undefined