public static Value selectChars(final ValueList values) {
        values.assertCheckCount(1, -1, false);
        final String value = values.getAsString(0, "");
        values.assertCheckTypes((v, i) -> i == 0 || v.isLongBetween(1, value.length()), i -> "valid index", false);
        final StringBuilder result = new StringBuilder();
        for (int i = 1 ; i < values.size(); i++) {
            result.append(value.charAt((int) (values.get(i).getAsLong()) - 1));
        }
        return Value.of(result.toString());
    }
Holds implementations for functions in Formula
main/src/main/java/cgeo/geocaching/utils/formulas/FormulaUtils.java