docs: function lpoisson(lambda) {
        return function(x) {
           if (lambda === 0) { return x === 0 ? 0 : -Infinity; }
           if (!utils.isFinite(lambda)) { return -Infinity; }
           if (x < 0) { return -Infinity; }
           if (x === 0) { return -lambda; }
           if (x === Infinity) { return -lambda + x * Math.log(lambda) - Infinity; }
           return -stirlerr(x) - bd0(x, lambda) - 0.5 * Math.log(C.twopi * x);
        };
     }