Date: Dec 4, 2012 4:09 AM
Author: Simons, F.H.
Subject: Mathematica 9: bug in merging cells

When writing a larger program, I usually test the single steps in 
separate cells, then merge these cells and localize the variables by
wrapping the contents of the merged cells in a Module. By doing so in
Mathematica 9, due to a bug in merging cells, I lost a lot of code. I
reported this bug to WRI two weeks ago. It is not yet repaired in the
new release, so I thought I would mention this annoying bug here as well.

This is an example (Mathematica 9 for Windows, I am not aware if this
happens for other systems too).

Construct two input cells, the first one containing

a=1;
(* comment *)
b=2;

and the second one

c=3;

When merging these two cells the contents ending with the comment is
shown in an orange rectangle, with the error message that there is a
RowBox with an invalid first argument. I cannot see any reason why this
merging of cells could not be undone with ctrl-Z, but it cannot and
therefore part of my code was gone.

In this simple example I could see how to fix it. Toggle ctrl-E for
seeing the box structure. Place a } just behind the RowBox with the
comment and a { behind the very first RowBox[, that is followed on the
next line with another RowBox.

During writing this message, I found that the same happens when you try
to copy and paste the first cell.

Fred Simons
Eindhoven University of Technology