  Cumulants linearize convolution of measures.
  We use a formula of Good to define noncommutative cumulants in a very general
setting.It turns out that the essential property needed is exchangeability of
random variables. Roughly speaking the formula says that cumulants are moments
of a certain ``discrete Fourier transform'' of a random variable. This provides
a simple unified method to understand the known examples of cumulants, like
classical, free cumulants and various q-cumulants.
