Strings s = Strings();
string[] elems = {"name=xxx","age=xx"};
string ret = s.join(elems, "&");
println(ret);
assertEqual(ret, "name=xxx&age=xx");

StringBuilder b = StringBuilder();
b.writeString("10");
b.writeString("20");
int l = b.writeString("30");
string s = b.String();
printf("s:%s, len=%d ",s,l);
assertEqual(s,"102030");
byte[] b2 = toByteArray("40");
b.WriteBytes(b2);
s = b.String();
assertEqual(s,"10203040");
println(s);

// tests whether the string s begins with prefix.
bool hasPrefix(string s, string prefix){
    byte[] bs = toByteArray(s);
    byte[] bp = toByteArray(prefix);    
    return len(bs) >= len(bp) && toString(bs[0:len(bp)]) == prefix;
}