Lining Up and PaddingTwo Similar Lists

[W. eWatson]

Maybe there's some function like zip or map that does this. If not, it's 
probably fairly easy to do with push and pop. I'm just checking to see if 
there's not some known simple single function that does what I want. Here's 
what I'm trying to do.

I have a list dat like (assume the items are strings even thought I'm 
omitting quotes.):
[a.dat, c.dat, g.dat, k.dat, p.dat]

I have another list called txt that looks like:
[a.txt, b.txt, g.txt, k.txt r.txt, w.txt]

What I need is to pair up items with the same prefix and use "None", or some 
marker, to indicate the absence of the opposite item. That is, in non-list 
form, I want:
a.dat a.txt
None  b.txt
c.dat None
g.dat g.txt
k.dat k.txt
p.dat  None
None  r.txt
None  w.txt

Ultimately, what I'm doing is to find the missing member of pairs.
-- 
            Wayne Watson (Watson Adventures, Prop., Nevada City, CA)

              (121.015 Deg. W, 39.262 Deg. N) GMT-8 hr std. time)
               Obz Site:  39° 15' 7" N, 121° 2' 32" W, 2700 feet

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