formatting ?: operator

[John W. Eaton]

While we're on the subject of formatting and whitespace, could we also 
choose a convention for the ?: operator when it is longer than one line?

For example, I typically write them like this:

  x = ((some_long_condition || some_other_longer_condition)
       ? some + expression - that / is * not + short
       : another + expression - that / is * also + not - short);

By including the extra parens around the whole expression, Emacs knows 
how to indent this as above (display with a fixed-width font).  I've 
noticed that we now have some files that use this convention instead:

  retval = lower ? octave::math::gammainc (x, a)
                 : 1.0 - octave::math::gammainc (x, a);

This looks fine, but I don't know how to get Emacs to do this for me. 
I'd rather be able to just hit TAB and have something reasonable happen. 
 If I ask Emacs to indent the above, I get

  retval = lower ? octave::math::gammainc (x, a)
    : 1.0 - octave::math::gammainc (x, a);

If I surround the expression with (), then Emacs will indent it as

  retval = (lower ? octave::math::gammainc (x, a)
            : 1.0 - octave::math::gammainc (x, a));

which is OK, but not really great.  By chance, this one lines up neatly, 
but that's not generally what will happen.  So that's why I would 
normally try to get the expression on one line or split before the ? and 
possibly the : if both expressions won't fit on one line.

jwe