a question about MultiSearcher

[王建新]

If I search in multiple dirs,I can use MultiSearcher.
The idf about a term is log(numDocs/(docFreq+1)+1).

In the two kinds of condition:
    1.only 1 dir to search
    2. 2 dirs to search
the idf about a term is not the same?

why not lucene consider giving a method to set the numDocs for class
IndexWriter?
just like:

        IndexWriter writer=new IndexWrite(......
        writer.setNumDoc(num1+num2);

Thanks.