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Part // interesting undocumented feature

• To: mathgroup at smc.vnet.net
• Subject: [mg122669] Part // interesting undocumented feature
• From: "Scot T. Martin" <smartin at seas.harvard.edu>
• Date: Sat, 5 Nov 2011 04:47:16 -0500 (EST)
• Delivered-to: l-mathgroup@mail-archive0.wolfram.com

A colleague new to Mathematica showed me something that I thought should cause an error:

Part[data[[All,1]]]

Of course, I thought the form should just be "data[[All,1]".

It turns out that there is form of Part that works like an identity function:

In[1]:= Part[5]
Out[1]= 5

Just out of a Mathematica trivia challenge question, can anyone think of an example when this undocumented syntax of Part[ ] could possibly be of use or why the programmers have it as an acceptable syntax?