Porting Range in Mozilla from IE

[Stefano]

Hi all,

I am porting these 2 instructions from IE to Mozilla 1.4
select_doc =document.selection;
range_doc=select_doc.createRange();

Mozilla has window.getSelection and document.createRange() so i have
thought
this:
var range_doc=document.createRange();
range_doc.setStart(active_el,range_doc.startOffset);
range_doc.setEnd(end_el,range_doc.endOffset);

where active_el and end_el are elements that specify about start and
end of selection.
I am obtaining errors over offset??
Why?

Can you help me?

Thanks Stefano