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This page last modified: Nov 02 2006

- Multivalued columns must be sub-elements. If a column contains a list
  or an array, that column must be a sub-element.

- Columns with a single value should be attributes, not elements.

- An element used as a container should always have the same set of
  sub-elements. If the set of sub-elements varies, then it is a
  different type of record and the containter element should have a
  different (unique, new) name to reflect the different purpose. If
  some sub-elements (columns) are optional, those sub-elements should
  ideally have a 'missing data' value, but could have an empty value
  (essentially null) instead. 

- Any column which will have sub elements must become an element (and
  cannot be an attribute).

For example a symposium with several sessions, each session has
several presenters. 

  <session title="a">
    <presenter name="ralf"/>
    <presenter name="bob"/>
    <presenter name="mary"/>
  <session title="b">
    <presenter name="pat"/>
    <presenter name="tad"/>
    <presenter name="jordan"/>