docs: 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)(); }
           };
        }