Haskell-report, chapter 3 - Expressions

[Steffen Mazanek]

Hello,

I do not completely understand the first part of chapter 3 of the 
Haskell-report.
Concretely I am stumbling about the notation of nonterminals indexed by 
their precedence
level. This should be a number ranging from 0 to 9. But what about this 
exp^{10}
production rule?
I would be very appreciated if someone could explain this part in some 
more detail.
I am trying to introduce a new language construct into this grammar and 
I want to
insert it at the right place :-)

Ciao,
Steffen