You can use round brackets "(" and ")", also square brackets "[" and "]" in expressions. They open earlier then ":" and "." operations execute.
Example,
<&data-guestbook:[mes*mes2]>
it is identical <&(data-guestbook:mes)*(data-guestbook:mes2)>.
For compariso, during processing
<&data-guestbook:(mes*mes2)>
at first elements mes*mes2 = <&data-guestbook:{}> multiply
However these are special features, and you may not use them at all. Especially useful situations when you use square brackets and factorials.
<&data-guestbook:[mes+mes2]!>
