t.Errorf(testname+" (fill 2): m == %d, expected %d", m, len(fus))
		}
		if err != nil {
			t.Errorf(testname+" (fill 3): err should always be nil, found err == %s", err)
		}
		s += fus
		check(t, testname+" (fill 4)", buf, s)
	}
	return s
}

// Fill buf through n writes of byte slice fub.
// The initial contents of buf corresponds to the string s;
// the result is the final contents of buf returned as a string.