How to align equal signs with limits?

[Etienne]

Hi! Consider the following code:

$ a &= b \
&=_(a + b + c) c $

It outputs this:

Because I wrote something below the second equal sign, the alignment with the previous one is made with respect to the content below the sign, but I would like the equal signs to be aligned regardless of what is beneath them. Is that possible?
Thanks in advance!

[Y.D.X]

Two options:

$ a &= b \
&stretch(=)_(a + b + c) c $

$ a &= b \
&=_#box(width: 0pt, box(width: 10em, $script(a + b + c)$)) #h(0.8em) c $

I believe the first one looks better, and the second one is your literal description.

[Etienne]

Thanks for your answer! I personally prefer the second option (up to aligning b and c but that’s easy), although the code is much more hacky. I think I will just avoid writing things under equal signs.