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?

The grammar is written using a kind of macro syntax, such that you can obtain the full grammar by expanding each of the rules for nonterminals with a superscript 'i' for each 'i' in the range 0 to 9. These rules then cover the grammar for expressions containing infix operators with precedences between 0 and 9. The '10' in the 'exp10' nonterminal doesn't refer to the precedence of an operator, it is there purely as a clue to to the reader that these expressions bind tighter than all the infix operators. Hope this helps. Cheers, Simon