What does "ans is no longer a built-in variable" mean?

[John W. Eaton]

On 23-Jun-2009, Judd Storrs wrote:

| I'm porting some matlab code to octave and I've run into a warning that can
| be reproduced thusly
| 
| octave:1> ans = 5
| warning: ans is no longer a built-in variable; please read the NEWS file or
| type `news' for details
| 
| I'm having trouble understand this warning message. I looked through the
| NEWS{,.1,.2,.3} files and I couldn't identify the relevant section. My first
| thought was that octave was warning me that octave doesn't assign the result
| of the last evaluation to "ans". But that doesn't seem to be the correct
| interpretation:
| 
| octave:1> sin(5)
| ans = -0.95892
| octave:2> ans
| ans = -0.95892
| 
| ...besides the error occurs when you assign a value to a variable
| called

It's a warning, not an error, isn't it?

| "ans", not when you try to use it. After thinking about the message
| obliquely for a bit, I decided that maybe octave was trying to warn me that
| my assigned a value to "ans" would override the automatic value for the rest
| of the scope or session. But that also doesn't seem to be the correct
| interpretation either:
| 
| octave:1> ans = 5
| warning: ans is no longer a built-in variable; please read the NEWS file or
| type `news' for details
| ans =  5
| octave:2> sin(5)
| ans = -0.95892
| octave:3> ans
| ans = -0.95892
| octave:4>
| 
| Does anyone know what this warning is about?

It may have been that when I converted all the built-in variables to
functions, I thought that ans should also be converted to a function.
But I don't think that is the correct thing to do.  I checked in the
following change.

  http://hg.savannah.gnu.org/hgweb/octave/rev/68f4034d4e7e

BTW, what is a good reason to assign a value to ans directly?

jwe