+JSON Map object containing subdomain names as its keys and domain
+objects as values. Element of the map with empty key "" has special
+meaning: the value of this map element is merged into the current
+domain object. This operaton happens first when a new domain object
+is analyzed, and is performed recursively. In the result of the
+merge, the `"map"` does not contain the element with empty key.
+Further operatons that can potentially modify the contents of the
+current domain object (`import` and `delegate` lookups) start when
+the empty element of the `"map"` has been recursively merged into
+the current object.