Any person compelled to appear, or who appears by request or permission of the Commission, in person at a formal investigative proceeding may be accompanied, represented and advised by counsel, as defined in
§ 201.101(a) of this chapter (
Rule 101(a) of the Commission's rules of practice): Provided, however, That all witnesses shall be sequestered, and unless permitted in the discretion of the officer conducting the investigation no witness or the counsel accompanying any such witness shall be permitted to be present during the examination of any other witness called in such proceeding.