Multidata Indicator (ie. McClellan Oscillator) EL Syntax Problem

[SaulFFeld@xxxxxxx]

Both TS 3.5 & 4 show the following EL code Syntax for the McClellan
Oscillator, applicable to stock market data: 

---------------------------------------------------------
Plot1(XAverage(Close of data1 - Close of data2,19) OF DATA1 - XAverage(Close
of data1 - Close of data2,39) OF DATA1,"McClellan");   [[1]]
--------------------------------------------------------

where using the Signal nomenclature, 
data1 could represent daily issues advancing, say as given by Symbol $ADV, and
data2 could represent daily issues declining, $DECL. 

The data for each Symbol can be placed in a Multidata Chart, 
on which the Indicator can be applied & it seems to work.

The syntax for the Exponential Average Function 
(page 270 of EL TS 4 Manual) is:
--------------------------------------------------
        Xaverage(Price,Length).      [[2]]
----------------------------------------------------

Since [[2]] is correct, & it is used in [[1]], 
[ where "Price" is replaced by "Close of data1 - Close of data2" ] 
why is the "OF DATA1" used TWICE in [[1]] instead of NOT AT ALL? 

I tried to delete those two "OF DATA1"s & the expression would not verify, 
it would highlight the "Xaverage" in the code, 
while giving following the error message:
GRAMMAR ERROR: Unresolvable Data Number.
If I add anything , like "OF DATA 35" & "OF DATA47" it does verify.

Nevertheless, the right values are being calculated by [[1]].

Is there some special undocumented Easy Language issue here because of
multidata streams?

May be I missed it, but does anyone know what is going on?

Whoever helps, thank you for it.

  -Saul