[erlang-questions] Trace erlang BIFs with dbg

[nato]

Are BIFs such as `length/1` part of the erlang module, or do BIFs live
somewhere else then get masqueraded ...

I am trying to trace functions like these with dbg, and nothing is yielded.
For example, I am trying the following with no success...

1> dbg:tracer(), dbg:p(all, c).
2> dbg:tpl(erlang, length, x).
3> erlang:length( [] ). %% or simply `length([]).`

Or, are my above dbg flags leaving the erlang module out... or... confused.