Actions now have a attribute called "method". Method has  enumerated values associated with them.. We need a way to 
define these values/options.

We could use the  brace []  since the curly braces are in use for delimiting attribute properties.

 > POST /compute/?action=stop HTTP/1.1
 > [...]
 > Category: stop; scheme="[...]"; class="action";
 > X-OCCI-Attribute: method=poweroff["graceful", "acpioff", "poweroff"]

With method undefined
 > POST /compute/?action=stop HTTP/1.1
 > [...]
 > Category: stop; scheme="[...]"; class="action";
 > X-OCCI-Attribute: method=["graceful", "acpioff", "poweroff"]