The Information Schema vs the PG Catalog

<div class="Section1"><p class="MsoNormal"><font face="Verdana" size="2"><span style="font-size:10.0pt;
font-family:Verdana">I’m writing PL/pgSQL routines that generate triggers, functions, and rules based on design
characteristicsof tables, columns, and other database objects.  These routines need to be able to look up the
definitionsof these objects.  I see that there are two places available to look up this info: the Information Schema
andin the PG Catalog.  </span></font><p class="MsoNormal"><font face="Verdana" size="2"><span style="font-size:10.0pt; 
font-family:Verdana"> </span></font><p class="MsoNormal"><font face="Verdana" size="2"><span style="font-size:10.0pt;
font-family:Verdana">Which source is preferable?  Or if that answer isn’t absolute, what are the reasons or conditions
forpreferring one over the other?  </span></font><p class="MsoNormal"><font face="Verdana" size="2"><span
style="font-size:10.0pt;
font-family:Verdana"> </span></font><p class="MsoNormal"><font face="Verdana" size="2"><span style="font-size:10.0pt;
font-family:Verdana">Also, a specific question:  Does the Information Schema offer any way to list the sequences that
existand their attributes?  I can’t seem to find any.</span></font><p class="MsoNormal"><font face="Verdana"
size="2"><spanstyle="font-size:10.0pt; 
font-family:Verdana"> </span></font><p class="MsoNormal"><font face="Verdana" size="2"><span style="font-size:10.0pt;
font-family:Verdana">~ TIA</span></font><p class="MsoNormal"><font face="Verdana" size="2"><span
style="font-size:10.0pt;
font-family:Verdana">~ Ken</span></font></div>