The hierarchy of a namespace is composed of nodes. Each node can have
children (called sub-nodes). Nodes are, in fact, ObjectNode.
Just like in a filesystem, nodes have different types.
ObjectNode can be either SubjectNode or
ActionNode. ObjectNode is an abstract data type,
which describes the namespace structure.
SubjectNode is a type for storing most Slide information.
A subject can be a file, a directory or a role (ie. user or group).
ActionNode is a type for defining available actions on Slide
objects. In Slide, actions can be dynamically placed in the namespace
to provide flexible security policy definition.