Logarithm of beta function
L = betaln(Z,W)
L = betaln(Z,W) computes
the natural logarithm of the beta function log(beta(Z,W)),
for corresponding elements of arrays Z and W,
without computing beta(Z,W). Since the beta function
can range over very large or very small values, its logarithm is sometimes
more useful.
Z and W must be real and
nonnegative. They must be the same size, or either can be scalar.
betaln(z,w) = gammaln(z)+gammaln(w)-gammaln(z+w)