code: function brcomp(a, b, x, y) {
        var lambda;
  
        lambda = a - (a + b) * x;
        if (Math.min(a, b) < 8) {
           return a * Math.log(x) + b * Math.log(y) - lbeta(a, b);
        }
        return 0.5 * (Math.log(a) + Math.log(b) - C.log2pi - Math.log(a + b)) -
               a * phi(1 - lambda / a) - b * phi(1 + lambda / b) -
               bcorr(a, b);
     }
docs: brcomp -- see p. 370 - 372 Computes log(x^a * y^b / B(a, b))