code: function(rate) {
           return {
              d: function(x, logp) { return dexp(rate, logp)(x); },
              p: function(q, lowerTail, logp) {
                 return pexp(rate, lowerTail, logp)(q);
              },
              q: function(p, lowerTail, logp) {
                 return qexp(rate, lowerTail, logp)(p);
              },
              r: function() { return rexp(rate)(); }
           };
        }
docs: Returns an object representing an exponential distribution with a
  given rate $\lambda > 0$,
  with properties `d`, `p`, `q`, `r`.
  ```
  expdistr(rate).d(x, logp)            // same as dexp(rate, logp)(x)
  expdistr(rate).p(x, lowerTail, logp) // same as pexp(rate, lowerTail, logp)(x)
  expdistr(rate).q(x, lowerTail, logp) // same as qexp(rate, lowerTail, logp)(x)
  expdistr(rate).r()                   // same as rexp(rate)()
  ```
  @memberof exponential