Allow some listener changes after init.
Allow some listener changes after init.

You’re not allowed to modify start event listeners after the transition is
scheduled, so as to avoid changes during a start event (due to changes being
copy-on-write). However, you should still be allowed to change the OTHER event
listeners (interrupt and end) before the transition starts; in particular,
transition.remove should be allowed during a start event!

d3/d3-transition