code: function bpser(a, b, x) {
        var c, ser;
  
        c = 1;
        ser = a * series(function(i) {
           if (i === 0) { return 0; }
           c = c * (1 - b / i) * x;
           return c / (a + i);
        });
        ser = ser > -1 ? log1p(ser) : -Infinity;
        return -lbeta(a, b) + a * Math.log(x) - Math.log(a) + ser;
     }
docs: bpser (7) Returns the log of the lower probability