Is an R sub-session somehow possible?

I wrote the interface between R and TeXmacs. Recently, I added tab
completion. However, there is one slight problem. In order to enable easy
interaction with R, I (I.e. my program) interact with the command-line
interface. This means that the user can invoke demo(), and then R will
interact with the user and ask to press enter.
It also means that the user can enter
a<-c(3,4
and then R will respond with '+'.

The problem is this: the way I implemented tab completion is calling an R
function that creates the completion. But, while in the middle of user
input, I can't call a function.

I guess that ESS for emacs has the same problem. when I enter "a<-c(3,4",
and then on the next line try to do tab completion, ESS replies that 'ESS
process is not ready. Finish your command before trying again'.
Of course while interacting with R on the command line, tab completion does
work.

So, the question is - is there any way to interrupt the current input to R,
call a function, get the return value, and then continue with the input
where it was? To do something similar to what pressing 'tab' in R does
internally? Something like the equivalent of ctrl-Z for a shell?

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