A ternary function. When applied to an instance of Cyc system string, an instance of non-negative integer, and an instance of positive integer, Substring Fn returns an instance of Cyc system string. (Substring Fn STRING START END) denotes the string that begins at position START of STRING and ends before position END. The first character of a string is at position 0. Thus, for example, (Substring Fn cycorp 0 2) denotes the string cy .