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list referencing with [[ ]][[ ]]


According to "the book" section 1.8.4,

t[[1]][[2]] "is equivalent to" t[[1,2]], "but is clumsier to write"

and that is all that I can find concerning the difference between the
two ways of refering to a list element.

But...

 In[37]:=
 alist = {{1,2},{3,4}}; alist[[2,1]] = 7 ; alist

 Out[37]=
 {{1,2},{7,4}}

So far so good, and yet...

 In[39]:=
 anotherlist = {{1,2},{3,4}}; anotherlist[[2]][[1]] = 7
 Set::"setps": 
    "\!\(anotherlist \\[LeftDoubleBracket] 2 \\[RightDoubleBracket]\) in
\
 assignment of part is not a symbol."
 Out[39]=
 7

What's the problem here?  Is there a difference between the two ways of
referencing list elements that "the book" neglects to mention?


--
--
"Perhaps my most deeply rooted hobby is to understand         Tim
Dellinger
 the mechanisms of simple and familiar                   
tdelling at uiuc.edu
 natural phenomena." Irving Langmuir     
http://www.ews.uiuc.edu/~tdelling


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