Constructs:

code embedded in output:
<%code> ++i; </%code>
% ++i;

output embedded in code:
int i=0;
<%output>whoa, it's <% i++ %>!</%output>

embed literal in the code:
#define URL "http://www.klever.net/"
<%literal>
 a text that is going to be there is a string
 with some possibilities <% URL %>
</%literal>