Historically, content models in SGML and XML 1.0 DTDs are required to be 1-unambiguous (or ‘deterministic’). Many assume this rule makes parsers simpler, but in fact the 1-unambiguity restriction has no practical utility; any benefits it may have can be obtained from other restrictions already present in schema language proposals like XML-Schema. In particular, these restrictions ensure that we can assign a type to an element using a SAX parser. That done, other operations such as document validation can also be done by a SAX parser. More importantly, the 1-unambiguity restriction provides no benefit for checking constraints such as key and key-reference validity. If we drop the 1-unambiguity constraint, other operations on XML become easier — especially type inference for query.