The notation E[P] is used to select items from the sequence obtained by evaluating
E. If the predicate P is numeric, the predicate selects an item if its
position (counting from 1) is equal to P; otherwise, the effective boolean value
of P determines whether an item is selected or not. The effective boolean value of a sequence
is false if the sequence is empty, or if it contains a single item that is one of: the boolean value
false, the zero-length string, or a numeric zero or NaN value. If the first item of the sequence is a node, or
if the sequence is a singleton boolean, number or string other than those listed above, the effective
boolean value is true. In other cases (for example, if the sequence contains two booleans or a date),
evaluating the effective boolean value causes an error.
In XPath 2.0, E may be any sequence, it is not restricted to a node sequence. Within
the predicate, the expression . (dot) refers to the context item, that is, the item
currently being tested. The XPath 1.0 concept of context node has thus been generalized, for example
. can refer to a string or a number.
Generally the order of items in the result preserves the order of items in E. As a
special case, however, if E is a step using a reverse axis (e.g. preceding-sibling), the
position of nodes for the purpose of evaluating the predicate is in reverse document order, but the
result of the filter expression is in forwards document order.
