Reading from and writing to the same URI (XTRE1500)

I'm having trouble understanding the restriction that
one cannot read from and write to the same URI during
a single transformation.  This prohibits the common (i
would think) 'use' case where one wants to overwrite a
file with a slightly modified version of the same
file.  For example, one wants to update a list of
contacts with one more contact.
 
To preempt the argument that ordering becomes an issue
(namely, did the read occur before or after the
write), I'll reiterate a sentiment posted in a thread
to this forum.  Put a stake in the ground and say that
all writes occur at the end of the transformation. 
All reads see the documents prior to modification.  Is
there a flaw in this reasoning that necessitated this
restriction? 
 
(Please note that I am *not* advocating writing to the
same document more than once in a single
transformation.  That inherently relies upon order of
execution.)
 
Steven



 
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