[antlr-interest] empty tree assertion rule

[whaefelinger]

Hi,
being a newbee to antlr and it's abilities I'm always bit afraid to
"forget" to visit parts of a generated AST when formulating rules on 
how to walk the tree within the tree parser.

So I came up with the idea to write a tree parser rule to ensure 
that given input tree is empty:

001 nullp
002 {
003    if( #nullp != null ) {
004     System.err.println("error: tree not empty!");
005     System.exit(1);
006    else {
007     return;
008    }
009 }
010    : #(INT INT) 
011    ;


This rule would then be used like this:

expr returns [float r]
{
    float a,b;
    a=b=0;
    r=0;
}
    :	#(PLUS a=expr b=expr nullp)    { r = a+b;  }
    ;

That means that my parser would generate an (unexpected)
expr like

 (+ 1 2 3)

I would  get  this  reported.

Of course the parser just generates this bad expr only 
if there's  something  wrong  with the grammar. So when
releasing the parser, the "nullp" rule should be removed
for efficency.

Now for the question:

Q1: does something like 'nullp' already exist and I'm just
    not aware of?

Q2: is there absolutly no need for 'nullp' because I missed
    the point? 







 
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