Compare two nodes.
The following construct(s) refer to this construct:
A NodeComp compares two nodes by their identity or by their
document order. The following rules hold:
Each operand must be either a single node or an empty sequence.
If either operand is an empty sequence the result is an empty sequence.
A comparison using the is operator yields
true if the two operand nodes have the same identity, otherwise
false is returned.
A comparison using the << operator yields
true if the left operand node precedes the right operand node in
document order, otherwise false is returned.
A comparison using the >> operator yields
true if the left operand node follows the right operand node in
document order, otherwise false is returned.
The result of this comparison is false, because each node
has its own identity (rule 3):
<a>42</a> is </a>
