XML Companion, The, Third Edition

Expression structure overview

It can be difficult to follow some of the labyrinthine ways in which XPath rules include each other. Yet there is a major chain of rules for expressions (Expr^{14}), as shown below. The order in which the following roles are linked (exclusively) to each other also indicates the order in which they are processed (so that, for example, '=' is evaluated after '<'):

{14} Expr           ... and ... or ... (adds nothing)
       |
 {21} OrExpr        ... and ...
					or
					...
        |
  {22} AndExpr    $a=$b
					and
					$y < $z ...
         |
   {23} EqualityExpr        $y < $z
					=
					$a < $b + $f * -2
          |
    {24} RelationalExpr               $a
					<
					$b + $f * -2
           |
     {25} AdditiveExpr                     $b
					+
					$f * -2
            |
      {26} MultiplicativeExpr                   $f
					*
					-2
             |
       {27} UnaryExpr                                -2
              |
        {18} UnionExpr    warning|chapter[9]/para     2
               |
         {19} PathExpr            chapter[9] ...