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+# UNO Services to Access the Configuration Database
+
+## Functional Overview
+
+This code parses the settings that are described in the `officecfg`
+directory, and provides a UNO API that code can use to set and get
+settings.
+
+## Source Overview
+
+    configurationprovider.cxx
+    configurationregistry.cxx
+    defaultprovider.cxx
+    services.cxx
+UNO service implementations.
+
+    access.cxx
+    childaccess.cxx
+    rootaccess.cxx
+UNO objects passed to clients.
+
+    components.cxx
+Central singleton that aggregates all data (reads in the XML files, manages
+modifications and global notifications).
+
+    groupnode.cxx
+    localizedpropertynode.cxx
+    localizedvaluenode.cxx
+    node.cxx
+    propertynode.cxx
+    setnode.cxx
+Internal representations of data nodes.
+
+    parsemanager.cxx
+    parser.hxx
+    valueparser.cxx
+    xcdparser.cxx
+    xcsparser.cxx
+    xcuparser.cxx
+    xmldata.cxx
+XML file reading.
+
+    modifications.cxx
+    writemodfile.cxx
+Modification management.
+
+    broadcaster.cxx
+Notification management.
+
+    additions.hxx
+    update.cxx
+Extension manager interface.
+
+    data.cxx
+    lock.cxx
+    nodemap.cxx
+    partial.cxx
+    path.hxx
+    type.cxx
+Utilities.
+
+
+## Some Implementation Notes
+
+### Mandatory Set Members
+
+- A set member can be marked as "mandatory," meaning that a member of that name
+must always be present in a set.
+
+- The above definition implies that calling replaceByName on a mandatory set
+member is OK.
+
+- The XCU format can contain `oor:mandatory` attributes on nodes.  (The XCS format
+does not support them.  In the `registrymodifications` file, `oor:mandatory`
+attributes should never be needed, as being mandatory cannot be controlled via
+the UNO API.)  The XCU reading code only evaluates the `oor:mandatory` attribute
+for set members, and ignores it everywhere else.
+
+- Only true sets support mandatory members.  A localized property for the "`*`"
+locale, though acting much like a set, does not support mandatory members.
+
+- The LibreOffice Registry Format document claims that group extension
+properties are implicitly mandatory, but at least the new configmgr code does
+not treat them like that (i.e., they can be removed again).
+
+- For simplicity, `setMandatory/getMandatory` are available as virtual functions
+at the base `Node`, even though they can only make sense for `GroupNodes` and
+SetNodes that are set members.  The default `getMandatory` implementation returns
+`NO_LAYER`, meaning `oor:mandatory` is not set to true in any layer.  (Returning
+`NO_LAYER` simplifies the code, e.g., `removeByName` does not have to check whether
+`getMandatory` is called on a member of a true set to decide whether to forbid
+removal.)
+
+- When committing changes (made through the UNO API), the "mandatory" status of
+inserted nodes must be updated (in case the insert is due to a `replaceByName`, or
+the "mandatory" flag was added by a concurrent modification of a lower layer).
+Also, for to-be-removed nodes, removal is ignored for (newly; due to concurrent
+modification of a lower layer) mandatory nodes (but still recorded in
+`registrymodifications`, so may take effect once the lower layer addition is
+removed again---whether or not that is a good idea).
+
+
+### XcuParser Modification Recording
+
+- `XcuParser` records modifications when reading user layer data
+(`valueParser_.getLayer() == Data::NO_LAYER`).
+
+- `oor:finalized and `oor:mandatory` attributes cannot be set via the UNO API, so
+it is assumed that user layer data does not contain them (for one, they are not
+written by `writeModFile`; for another, the logic to record modifications expects
+a `locprop(modify,fuse)` to be followed by one or more `value(fuse,remove)`, which
+would not necessarily be true if the `locprop` were only present in the user layer
+data to flag it as finalized).
+
+- The logic to record modifications considers the top of the XML element stack.
+In the following list of all possible cases, items marked with an asterisk are
+recorded:
+
+        ... group(modify,fuse) - group(modify,fuse) - ...
+        ... group(modify,fuse) - set(modify,fuse) - ...
+        ... group(modify,fuse) - *prop(modify,fuse,replace) - value(fuse)
+        ... group(modify,fuse) - *prop(remove)
+        ... group(modify,fuse) - locprop(modify,fuse) - *value(fuse)
+        ... group(modify,fuse) - locprop(modify,fuse) - *value(remove)
+        ... group(modify,fuse) - *locprop(replace) ...
+        ... set(modify,fuse,replace) - group(modify/fuse) - ...
+        ... set(modify,fuse,replace) - *group(replace/fuse) - ...
+        ... set(modify,fuse,replace) - *group(remove)
+        ... set(modify,fuse,replace) - set(modify/fuse) - ...
+        ... set(modify,fuse,replace) - *set(replace/fuse) - ...
+        ... set(modify,fuse,replace) - *set(remove)
+        Legend:
+            "...": zero or more further items
+            "- ...": one or more further items
+            "modify,fuse" etc.: any of those operations
+            "modify/fuse": a modify or a fuse on an existing member
+            "replace/fuse": a replace or a fuse on a non-existing member