Combine node sequences.
The following construct(s) refer to this construct:
A UnionExpr combines two node sequences by returning a
sequence in document order with all the nodes that occur in either of both
sequences. Duplicate nodes from the result sequences based on node identity are
eliminated.
You can use one of the operands union and |:
the effect is the same.
The following query returns the sequence (<A/>,
<B/>)
<A/> union <B/>
The following query returns the sequence (<B/>,
<B/>), since the element constructors create two element nodes
with the same name, but different identity:
<B/> union <B/>
