be <i>exact</i> or <i>loose</i>.
As unification recursively descends a composite type structure,
the matching mode used for elements of the type, the <i>element matching mode</i>,
remains the same as the matching mode except when two types are unified for
<a href="#Assignability">assignability</a> (<code>≡<sub>A</sub></code>):
in this case, the matching mode is <i>loose</i> at the top level but