strl := len(str)
		// short-circuit empty strings
		if xfixl == 0 || strl == 0 {
			return ""
		}
		// maximum possible length
		maxl := xfixl
		if strl < maxl {
			maxl = strl
		}
		// compare letters
		if pre {
			// prefix, iterate left to right
			for i := 0; i < maxl; i++ {
				if xfix[i] != str[i] {
					xfix = xfix[:i]
					break
				}
			}
		} else {
			// suffix, iterate right to left