Example:

{
  :f \o [ 1 2 ]
} = a_form

a_form.f

Stack contains: 2

{
  :f \o [ 1 2 ]
} = a_form

a_form,f !

Stack contains: 1 2

I'm not sure whether this is by design, as I didn't find any mention of it in the documentation, but this wasn't the behaviour I was expecting.