function substr( f_string, f_start, f_length ) {	// Return part of a string
	// 
	// +	 original by: Martijn Wieringa

	if(f_start < 0) {
		f_start += f_string.length;
	}

	if(f_length == undefined) {
		f_length = f_string.length;
	} else if(f_length < 0){
		f_length += f_string.length;
	} else {
		f_length += f_start;
	}

	if(f_length < f_start) {
		f_length = f_start;
	}

	return f_string.substring(f_start, f_length);
}