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<h1>Filter expressions</h1>
<p>The notation <code>E[P]</code> is used to select items from the sequence obtained by evaluating
<code>E</code>. If the predicate <code>P</code> is numeric, the predicate selects an item if its
position (counting from 1) is equal to <code>P</code>; otherwise, the <i>effective boolean value</i>
of <code>P</code> 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.</p>
<p>In XPath 2.0, <code>E</code> may be any sequence, it is not restricted to a node sequence. Within
the predicate, the expression <code>.</code> (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
<code>.</code> can refer to a string or a number.</p>
<p>Generally the order of items in the result preserves the order of items in <code>E</code>. As a
special case, however, if <code>E</code> 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.</p>
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