><a href="sub-sub-sealevel-method.html">[[,sealevel-method</a></code> operator can also
be used to retrieve items from within the <code>data</code> and <code>metadata</code> slots.
For example, <code>o[["temperature"]]</code> can be used to retrieve temperature
from an object containing that quantity. The rule is that a named
quantity is sought first within the object's <code>metadata</code> slot,
with the <code>data</code> slot being checked only if <code>metadata</code> does not
contain the item. This <co