[antlr-interest] added pattern rewriting example

[Terence Parr]

On Nov 30, 2008, at 1:07 PM, Oliver Zeigermann wrote:

> OK. I do not quite grok it. Looking at
>
> topdown
>    :   ^('*' INT ^(VEC (e+=.)+)) -> ^(VEC ^('*' INT $e)+)
>    ;
>
> bottomup
>    :  ^('*' a=. b=INT {$b.int==0}?) -> $b // x*0 -> 0
>    ;
>
> Why has the second to be in bottomup?

x*(4*0)

If you do it top down, you won't see x*0, you'll see only x*(4*0).   
You will rplace 4*0 with 0, but then you won't see x*0.  You need to  
do it innnermost (bottomup) not outermost (topdown) to see all  
opportunities.

You need to do

4*0 -> 0
x*0 -> 0

not

4*0 -> 0

and stop.

Ter