code: function(a, s) {
           return {
              d: function(x, logp) { return dgamma(a, s, logp)(x); },
              p: function(q, lowerTail, logp) {
                 return pgamma(a, s, lowerTail, logp)(q);
              },
              q: function(p, lowerTail, logp) {
                 return qgamma(a, s, lowerTail, logp)(p);
              },
              r: function() { return rgamma(a, s)(); }
           };
        }
docs: Returns an object representing a gamma distribution, with properties `d`, `p`, `q`, `r`.
  ```
  gammadistr(a, s).d(x, logp)            // same as dgamma(a, s, logp)(x)
  gammadistr(a, s).p(x, lowerTail, logp) // same as pgamma(a, s, lowerTail, logp)(x)
  gammadistr(a, s).q(x, lowerTail, logp) // same as qgamma(a, s, lowerTail, logp)(x)
  gammadistr(a, s).r()                   // same as rgamma(a, s)()
  ```
  @memberof gamma