Select nodes on a path.
The following construct(s) refer to this construct:
A PathExpr creates a sequence of nodes by following a path
from a starting point that is specified either by:
an absolute path expression, which has / as its first
character and starts at the context node's root node, or by:
a relative path expression, which starts at the context node.
See AbsolutePathExpr and RelativePathExpr for examples.
