function toJSON() {
			var out;
			var i;
			if ( this !== tuple ) { // eslint-disable-line no-invalid-this
				throw new TypeError( 'invalid invocation. `this` is not host tuple.' );
			}
			out = {};
			for ( i = 0; i < nfields; i++ ) {
				out[ fields[i] ] = tuple[ indices[i] ];
			}
			return out;
		}
Serializes a tuple as JSON.

@private
@memberof tuple
@throws {TypeError} `this` must be the host tuple
@returns {JSON} tuple JSON representation