When Bonds are Gone

"Buy 'em."
--Louis Winthorpe III (Trading Places)

Yesterday the ECB announced that it would cut its deposit rate to -.5% (!) and reboot it bond buying program (a.k.a. 'quantitative easing') at the rate of 20 billion euros per month.

Beyond the insanity of negative interest rate policy (NIRP) that these actions reflect, there is growing concern that the ECB will soon run out of bonds to purchase. The ECB will essentially have cornered the bond market--with money printed out of thin air, no less.

When bonds are gone, then what? Fill in the blank,

When a central bank runs out of bonds to buy, it will then turn to buying _____.

Incidentally, ____ are higher on the news.