function set( symbol ) {
	/* eslint-disable no-invalid-this */
	if ( indexOf( SYMBOLS, symbol ) === -1 ) {
		throw new TypeError( format( 'invalid assignment. `%s` must be a supported symbol. Symbols: "%s". Value: `%s`.', 'symbol', SYMBOLS.join( '", "' ), symbol ) );
	}
	debug( 'Current value: %d.', this._symbol );

	this._symbol = symbol;
	debug( 'New Value: %d.', this._symbol );

	this.emit( 'change' );
}
Sets the symbol.

@private
@param {string} symbol - symbol
@throws {TypeError} must be a supported symbol