\subsection{Comment on Exclusive Non-Leptonic Decays}
%
%
%
%
The effective weak Hamiltonian mediating nonleptonic decays
consists of four quark operators, the matrix elements of which 
need to be calculated to obtain the amplitude for an exclusive 
nonleptonic decay. In constructing the effective hamiltonian 
from the standard model of weak interaction one integrates out the 
heavy weak bosons and by the usual machinery of effective theory one 
may calculate rediative corrections to resum logarithms of the 
mass $M_W$ of the weak boson. These corrections are known to
next-to-leading logarithmic precision \cite{BurasNLLA}.   

At the scale of the bottom quark one may apply 
heavy quark effective theory to access also the logarithms of
the bottom quark mass $m_b$ and even of the charm mass $m_c$, 
both treated as perturbative scales. These corrections are known to 
leading logarithmic accuracy \cite{GKMW}.  

At present no theoretically clean method exists to evaluate these 
matrix elements. Furthermore, the restrictions due to symmetries 
(heavy quark symmetries as well as the symmetries of the light 
degrees of freedom) do not restrict these decays as it is the case
for exclusive semileptonic decays \cite{MRRnonlep} and hence the 
heavy mass limit has not added anything to our knowledge concerning 
exclusive nonleptonic decays. 

In order to obtain some numbers to compare to data one commonly 
used approach is the so called factorization assumption. It has 
been discussed in \cite{DGfact} that for specific decays this 
assumption may hold, but in general factorization is completely 
ad hoc and may even be shown not to hold in the general case. 

This assumption consists of rewriting the matrix element of
the four quark operator as a product of two matrix elements of 
quark currents. Once this assumption is made, one may relate the 
the exclusive nonleptonic decays to the corresponding semileptonic 
ones and the meson decay constants \cite{BSWnonlep}.

Under this assumption one may thus make use of heavy quark symmetry
as it restricts the form factors of semileptonic decays. We shall 
not discuss any details here since anything that can be said under the 
assumption of factorization is already covered in the sections on
semileptonic decays. 

