code: function dpois(lambda, logp) {
        logp = logp === true;
  
        return function(x) {
           if (utils.hasNaN(x, lambda) || lambda < 0) { return NaN; }
           if (x !== Math.floor(x) || utils.isInfinite(x)) {
              return logp ? -Infinity : 0;
           }
  
           return logp ? lpoisson(lambda)(x) : Math.exp(lpoisson(lambda)(x));
        };
     }
docs: Evaluates the Poisson probability mass function at `x`:
  $$\textrm{dpois}(\lambda)(x) = \frac{\lambda^x}{x!}e^{-\lambda}$$
  
  Expects $\lambda > 0$.
  
  `logp` defaults to `false`; if `logp` is `true`, returns the
  logarithm of the result.
  @fullName dpois(lambda, logp)(x)
  @memberof poisson