Dave VanHorn wrote:
>
> > >  You may be able to prevent syntax error mechanicaly by keying the
> blocks
> > > such that a connection cannot be made between two operators for example.
> > > This would simplify the code.  On the other hand, you may deliberately
> > want
> > > to allow syntax errors to improve the learning content of the toy.
> >
>  Tricky,  7 = 8 + -1 is legal.

Then add "bracket blocks".  My algebra teachers would have made me
write:
  7 = 8 + (-1)

At what age do school kids start dealing with negative numbers?  (Andy?
You're the kid expert here.)  My guess would be sometime after they lose
interest in playing with blocks ;)

--Matt