A node-list will typically be implemented in a lazy fashion. In other
words, the internal representation of a node-list is not a list
of nodes, but a representation of the specification that constructed
the node-list. For example, if an application uses the
node-list-count procedure on a node-list, it would be
inefficient to build the node-list, count it, and then discard the
node-list; it would be better simply to count how many distinct nodes
match the node-list's specification.
A node-list with a single member is referred to as a singleton
The named-node-list data type is a subtype of the
node-list data type that represents a node-list each of
whose members has a string-valued property that uniquely identifies
the node in the node-list.
nl is used for an argument that shall be a node-list.
snl is used for an argument that shall be a singleton node-list.
nnl is used for an argument that shall be a named-node-list.