It is possible to append an arbitary number of quote symbols
at the end of an prefix/infix/mixfix operator:

            applied form      standalone form

              -' 42               (-'_)
              x +' y              (+')
              a[0]' <- 1          ([]'<-)

Pretty-printing will use the quote symbols for disambiguation.

The derived symbols can be produced by Why3 by appending
a suffix of the form "_toto" or "'toto". These symbols can
be parsed/printed as "(+)_toto" or "(+)'toto", respectively.