  If a hadronic production process is dominated by two body final state
interactions, the amplitude $A$ for the production process can be expanded as a
sum of the scattering amplitudes $T$ for the relevant two body channels. Van
Beveren and Rupp have claimed that the unitarity relation ${\rm {Im}} A= T^\dag
A$ can be satisfied if the coefficients in this expansion are complex. We
demonstrate that the coefficients have to be real if the scattering amplitudes
$T$ satisfy unitarity. Van Beveren and Rupp have merely written real
coefficients as a sum of complex numbers.
