Take the 2-minute tour ×  I cannot understand how Mathematica
manages levels, and so it's always a painful try-and-fail to use
Flatten. Can someone please give me a very clear definition?  If
you feel like giving an example, please tell me how to turn this
list  {{a, {a1}}, {b, {b1}}, {c, {c1}}, ...}   {{a, a1}, {b,
b1}, {c, c1}, ...}  with Flatten, if possible.  share|improve
this question I discussed levels briefly on my book, here - this
might be helpful. –  Leonid Shifrin Dec 2 '12 at 15:07 possible
duplicate of Flatten command: matrix as second argument – 
rcollyer Dec 2 '12 at 15:09 @rcollyer May be this is not a dupe
given the wider context of the initial question. It might be a
little open-ended though. –  Leonid Shifrin Dec 2 '12 at 15:10
well, there are lots of questions on stack exchange whose answer
is a sort of handbook/definition. Since I couldn't find anything
on the Internet, this could be very well a place to put
something clear. About the example, I see that it's not possible
to do with a si