function Ext_GetMainIdx() {
		if ( ($this->ExtInfo!==false) && isset($this->ExtInfo['main']) ) {
			return $this->FileGetIdx($this->ExtInfo['main']);
		} else {
			return false;
		}
	}
function Ext_GetMainIdx() {
		if ( ($this->ExtInfo!==false) && isset($this->ExtInfo['main']) ) {
			return $this->FileGetIdx($this->ExtInfo['main']);
		} else {
			return false;
		}
	}
Return the idx of the main document, if any.