normalize! not exported + not defined for non-probabilistic case

[THinnerichs]

1. normalize! is not exported but is crucial when handling probabilities.
2. When handling non-probabilistic grammars, normalize! should throw an error, e.g. for the following grammar:

grammar = @cfgrammar begin
           Number = |(1:2)
           Number = x
           Number = Number + Number
           Number = Number * Number
       end

[ReubenJ]

Why should it fail on non-probabilistic grammars? Shouldn't it return the grammar, unchanged?