Multiple LaTeX classes in a layout

[Marshall Feldman]

Hello,

I'm writing my first LyX layout and running into trouble. When I use the 
following command in the layout file, the document class appears in the 
Document Settings drop-down list:

   #  \DeclareLaTeXClass[letter,graphix]{letter (mine)}

but the letter does not format correctly yet because I need to be using 
my own letter class name, myletter.cls, but haven't gotten that far. 
When I try the following, LyX no longer displays the class as available.

   #  \DeclareLaTeXClass[letter,graphix,calc]{letter (mine)}

What am I doing wrong? Is there an example somewhere of a layout that 
requires multiple LaTeX classes?

Thanks.

    Marsh Feldman