Business trip DTD

<!ELEMENT business-trip (
    airport,
    (office,drink?,dinner,drink+,shower,sleep,breakfast?)+,
    airport
)>

There were efforts to somehow fit a new element, sightseeing into a revised specification, and specify it in the business-trip's content with an optional quantifier, however usability research has shown that real-world documents would almost never contain an instance of it, mainly due to external forces that work to keep the document size at minimum.