src/main/java/org/xmlet/xsdparser/xsdelements/XsdAttribute.java
private void rule3() {
        if (attributesMap.containsKey(REF_TAG) && (simpleType != null || form != null || type != null)){
            throw new ParsingException(XSD_TAG + " element: If " + REF_TAG + " attribute is present, simpleType element, form attribute and type attribute cannot be present at the same time.");
        }
    }
private void rule3() {
        if (attributesMap.containsKey(REF_TAG) && (simpleType != null || form != null || type != null)){
            throw new ParsingException(XSD_TAG + " element: If " + REF_TAG + " attribute is present, simpleType element, form attribute and type attribute cannot be present at the same time.");
        }
    }
Asserts if the current object has a ref attribute at the same time as either a simpleType as children, a form attribute or a type attribute.
Throws an exception in that case.