Return a string whose value is the substring starting at the optionally specified position in the specified string, of length the optionally specified numChars or length() - position, whichever is smaller. If position is not specified, 0 is used (i.e., the substring is from the beginning of this string). If numChars is not specified, k_NPOS is used (i.e., the entire suffix from position to the end of the string is returned). The behavior is undefined unless position is within the string boundaries (0 <= position <= string.length()).