code: function dgamma(a, s, logp) {
        logp = logp === true;
  
        return function(x) {
           return logp ? dgammaLog(a, s)(x) : Math.exp(dgammaLog(a, s)(x));
        };
     }
docs: density function 1 / (s^a * gamma(a)) * x ^ (a - 1) * exp(-x / s) 
  Evaluates the Gamma density function at `x`:
  $$\textrm{dgamma}(a, s)(x) = \frac{1}{s^a\Gamma(a)}x^{a-1}e^{-x/s}$$
  
  Expects $a > 0$, $s > 0$, and $x > 0$.
  
  `logp` defaults to `false`; if `logp` is `true`, returns the
  logarithm of the result.
  
  @fullName dgamma(a, s, logp)(x)
  @memberof gamma