  The exponentials \(\bang\) and \(\quest\) in linear logic are less carved in the marble than the other connectives. Indeed, if one uses traditional sequent calculus presentations, the exponentials are not canonical in the following sense: if you introduce another copy of exponentials, say \(\bang'\) and \(\quest'\), with the same rules as the original ones, there is no way to prove that \(\bang\) is equivalent to \(\bang'\), and \(\quest\) to \(\quest'\), while for the other connectives this is easily established.  Various applications of non-canonical exponentials can be found in (Danos et. al., 1993;  Nigam &amp; Miller, 2009).  Although the decidability of MELL is currently being debated  (Bimbó 2015, Straßburger forthcoming),  extending MLL with three pairs of \(\bang\) and \(\quest\) yields a logic that is undecidable  (Chaudhuri 2018).  Martini and  Masini 1995 describe a “2-sequent”  proof system in which the exponentials are canonical.