A group of selectors object is represented as a reference to a Perl array, which contains zero or more selector object.
$group_of_selectors = [$selector1, $selector2,
..., $selectorsn];
A selector object is represented as a 
reference to a Perl array, which contains zero or more sequence of a 
combinator value followed by a 
sequence of simple selectors object.
The first combinator value, if any, is always 
(and MUST be) 
DESCENDANT_COMBINATOR.
$selector = [$combinator1, $sss1, 
$combinator2, $sss2, ..., 
$combinatorn, $sssn];
A combinator value is one of combinator types.
A sequence of simple selectors pbject is represented as a reference to a Perl array, which contains zero or more simple selector object.
$sss = [$ss1, $ss2, ..., 
$ssn];
A simple selector object is one of following objects:
Represented as a reference to a Perl array, which contains a value of
    NAMESPACE_SELECTOR,
    followed by a scalar value.  The scalar value is either 
    undef, which represents the null namespace (encoded as an
    empty namespace prefix in Selectors), or a non‐empty string, which
    represents the namespace URI of an element.
$selector = [NAMESPACE_SELECTOR, undef]; ## null namespace
$selector = [NAMESPACE_SELECTOR, $nsuri]; ## non-null namespace
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Copyright  
Wakaba
<wakaba@suikawiki.org>
This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.