  In neutral meson mixing, a certain class of convolution integrals is required
whose solution involves the error function $\mathrm{erf}(z)$ of a complex
argument $z$. We show the the general shape of the analytic solution of these
integrals, and give expressions which allow the normalisation of these
expressions for use in probability density functions. Furthermore, we derive
expressions which allow a (decay time) acceptance to be included in these
integrals, or allow the calculation of moments.
  We also describe the implementation of numerical routines which allow the
numerical evaluation of $w(z)=e^{-z^2}(1-\mathrm{erf}(-iz))$, sometimes also
called Faddeeva function, in C++. These new routines improve over the old
CERNLIB routine(s) WWERF/CWERF in terms of both speed and accuracy. These new
routines are part of the RooFit package, and have been distributed with it
since ROOT version 5.34/08.
