Element planner is the root element of the document, which contains any number
of optional year elements. The declarations (lines 6 and 7) specify that year element
must contain one or more date elements and must contain an attribute value that has
character data.
Line 9 indicates that a date element contains one or more note elements. Element
date is required to have two attributes: month and day. Element note contains parsed
character data and an optional attribute time.