Negation

[Christian Maeder]

> | I imagine it would be something like deleting the production
> | 
> |     lexp6    ->      - exp7

The rational for the current choice was the example:

f x = -x^2

> | and adding the production
> | 
> |     exp10    ->      - fexp

But I would also recommend this change.

It would also make sense to allow "-" before "let", "if" and "case" or
another "-" expression, but that's a matter of taste.

Cheers Christian