xpath

XPath defines two terms—context node-set and context node— to help describe
how the location path evaluation process works. The context node-set is

defined as the current set of nodes that has been identified up to a given point in

the expression. The context node is defined as the current node being processed.

XPath defines several abbreviations 

XPath supports standard and/or boolean expressions. Each operand is evalu-

ated and coerced to a boolean (as if by calling the boolean function) before

evaluation.

To select an attribute using unabbreviated syntax, we can write the following:
attribute:: attributename
Or, in the abbreviated form, simply write the following:
@attributename

xpath="author"
Selects the child elements named author that do not belong to any namespace.

xpath="author|character"

Selects the child elements named author or character that do not belong to any

namespace.

xpath="lib:author"

Selects the child elements named author that belong to the namespace whose

prefix is "lib".

xpath="*"

Selects all the child elements.

xpath="lib:*"

Selects all the child elements that belong to the namespace whose prefix is "lib".

xpath="authors/author"

Selects all the authors/author child elements.

xpath=".//author"

Selects all the elements that are descendants of the current node, named author,

and don't belong to any namespace.

xpath="author/@id"

Selects the id attribute of the author child element (allowed only for xs:field,

and not for xs:selector).

xpath="@id|@name"

Selects @id or @name (valid only in xs:field, since attributes are forbidden in

xs:selector).

The following are forbidden:

xpath="/library/author"

Absolute paths are not allowed.

xpath="../author"

The parent axis is not allowed.