docs: function bup(a, b, x, y, n) {
        var logSeries;
  
        logSeries = Math.log(series(function(i, v) {
              if (i === 0) { return 1; }
              return v * x * (1 + (b - 1) / (a + i));
           }, n));
        return a * Math.log(x) + b * Math.log(y) - lbeta(a, b) - Math.log(a) +
           logSeries;
     }