Which map is active when I overwrite text with R

[Titus von der Malsburg]

In insert mode, I bind TAB to a custom expand function that is based
on hippie-expand.  However, when I overwrite text with the R command,
I would also like to be able to use the expand function.  During
overwriting, TAB is bound to indent-for-tab-command and unfortunately
it's not clear to me which map is active during overwrite.  Could
somebody please give me a hint?

Thanks!
  Titus