docs: function dexp(rate, logp) {
        logp = logp === true;
  
        if (utils.hasNaN(rate) || rate < 0) { return function(x) { return NaN; }; }
  
        return function(x) {
           if (rate === Infinity) { return NaN; }
           if (x < 0) {
              return logp ? -Infinity : 0;
           }
           return logp ? -rate * x + Math.log(rate)
                       : rate * Math.exp(-rate * x);
        };
     }