added double rewrite to context macro

git-svn-id: http://acl2-books.googlecode.com/svn/branches/6.3@2140 303c5b2c-9740-0410-b014-fd8f9db7be32
added example of using nary to extend ACL2 congruence capabilities

git-svn-id: http://acl2-books.googlecode.com/svn/branches/6.3@2143 303c5b2c-9740-0410-b014-fd8f9db7be32