Write a function called substring() to extract a portion of a character string. The function should be called as follows: substring(source, start, count, result); where source is the character string from which you are extracting the substring, start is an index number into source indicating the first character of the substring, count is the number of characters to be extracted from the source string, and result is an array of characters that will contain the extracted substring. Please refer to Kochan, Chapter 9, problem 4 for more details.