124 Chapter 3 XML Syntax and Parsing Concepts as  shown,  the  only  restriction  is  that  there  can  be  no  space  between  the  initial question mark and the target. Some examples follow. <?xml-stylesheet type="text/xsl" href="foo.xsl" ?> <?MortgageRateHandler rate="7%" period="30 years" ?> <?javaApp class="MortgageRateHandler" ?> <?javaApp This is the data for the MortgageRateHandler, folks! ?> <?acroread file="mortgageRates.pdf" ?> Processing instructions are not part of the actual structure of the document, so they may appear almost anywhere, except before the XML declaration or in a CDATA section. The parsers responsibility is merely to pass the PI and its data on to the application. Since the same XML document could be processed by multiple appli- cations, it is entirely possible that some applications will ignore a given PI and just pass it down the chain. In that case, the processing instruction will be acted upon only by the application for which it is intended (has meaning). Although an XML declaration looks like a processing instruction because it is wrapped in the delimiters "<?" and "?>", it is not considered a PI. It is simply an XML declaration, the one-of-a-kind markup that may or may not be the first line of the document. The  target  portion  of  the  processing  instruction  can  be  a  notation  (defined  in chapter 4). For example: <!NOTATION AcrobatReader SYSTEM "/usr/local/bin/acroread"> The corresponding PI would be: <?AcrobatReader file="Readme.pdf" size="75%" ?> Entity References Entity  references  are  markup  that  the  parser  replaces  with  character  data.  In HTML,  there  are  hundreds  of  predefined  character  entities,  including  the  Greek alphabet, math symbols, and the copyright symbol. There are only five predefined entity references in XML, however, as shown in Table 3-2. TABLE 3-2  Predefined Entity References Character Entity Reference Decimal Representation Hexidecimal Representation < &lt; &#60; &#x3C; > &gt; &#62; &#x3E; & &amp; &#38; &#x26; " &quot; &#34; &#x22; ' &apos; &#39; &#x27; sall03.fm  Page 124  Wednesday, April 24, 2002  11:34 AM