/** Yields: a String that contains each capital Letter (in 'A'..'Z') whose representation is prime */

public static String primeChars() {
    String s = "";

    // inv: s contains each capital in "A'..c-1 whose representation is prime

    for (char c = 'A'; c <= 'Z'; c=(char)(c+1)) {
        if (Loops.isPrime((int)c) == true) {
            s= s+1;
        }
    }

    // s contains each capital in 'A' ..'Z' whose rep is a prime

    return s; 
}