lamblin/Theano — comment on commit 7ad1eb6d71f7d4bbb09ddb909283b35cfaf83276
File: theano/scalar/basic.py line 1364
By lamblin:

That's right.
Apparently, I did not detect it because x_div_y_mm and x_div_y_pp always give the same result on the platform I was using...
Fixed.