@Override
    public final boolean isValid(String literal, ValidationContext context) {
        try {
            checkValid(literal);
        } catch (DatatypeException e) {
            return false;
        }
        return true;
    }
Calls <code>checkValid(CharSequence literal)</code>.
@param literal the value
@param context the validation context (ignored by subclasses)
@return <code>true</code> if valid and <code>false</code> if not
@see org.relaxng.datatype.Datatype#isValid(java.lang.String, org.relaxng.datatype.ValidationContext)
src/nu/validator/datatype/AbstractDatatype.java