ach condition type specifies a set of field names. A condition
belonging to a condition type includes a value for each of the type's
field names. These values can be extracted from the condition by using
the appropriate field name.</p>
<p>There is a tree of condition types with the distinguished <code>&amp;condition</code>
 as its root. All other condition types have a parent condition type.</p>
<p>A condition belonging to several condition types with a common
supertype may have distinct values for the supe