The multinomial distribution is a generalization of the binomial
distribution to multiple categories. It is perhaps easiest to think
that we first extend a `dist_bernoulli()`

distribution to include more
than two categories, resulting in a `dist_categorical()`

distribution.
We then extend repeat the Categorical experiment several (\(n\))
times.

`dist_multinomial(size, prob)`

We recommend reading this documentation on https://pkg.mitchelloharawild.com/distributional/, where the math will render nicely.

In the following, let \(X = (X_1, ..., X_k)\) be a Multinomial
random variable with success probability `p`

= \(p\). Note that
\(p\) is vector with \(k\) elements that sum to one. Assume
that we repeat the Categorical experiment `size`

= \(n\) times.

**Support**: Each \(X_i\) is in \({0, 1, 2, ..., n}\).

**Mean**: The mean of \(X_i\) is \(n p_i\).

**Variance**: The variance of \(X_i\) is \(n p_i (1 - p_i)\).
For \(i \neq j\), the covariance of \(X_i\) and \(X_j\)
is \(-n p_i p_j\).

**Probability mass function (p.m.f)**:

$$ P(X_1 = x_1, ..., X_k = x_k) = \frac{n!}{x_1! x_2! ... x_k!} p_1^{x_1} \cdot p_2^{x_2} \cdot ... \cdot p_k^{x_k} $$

**Cumulative distribution function (c.d.f)**:

Omitted for multivariate random variables for the time being.

**Moment generating function (m.g.f)**:

$$ E(e^{tX}) = \left(\sum_{i=1}^k p_i e^{t_i}\right)^n $$

stats::Multinomial

```
dist <- dist_multinomial(size = c(4, 3), prob = list(c(0.3, 0.5, 0.2), c(0.1, 0.5, 0.4)))
dist
#> <distribution[2]>
#> [1] Multinomial(4)[3] Multinomial(3)[3]
mean(dist)
#> [,1] [,2] [,3]
#> [1,] 1.2 2.0 0.8
#> [2,] 0.3 1.5 1.2
variance(dist)
#> [,1] [,2] [,3]
#> [1,] 0.84 1.00 0.64
#> [2,] 0.27 0.75 0.72
generate(dist, 10)
#> [[1]]
#> [,1] [,2] [,3]
#> [1,] 1 3 0
#> [2,] 2 1 1
#> [3,] 1 3 0
#> [4,] 1 3 0
#> [5,] 0 4 0
#> [6,] 1 2 1
#> [7,] 1 3 0
#> [8,] 4 0 0
#> [9,] 1 2 1
#> [10,] 2 2 0
#>
#> [[2]]
#> [,1] [,2] [,3]
#> [1,] 1 1 1
#> [2,] 1 2 0
#> [3,] 0 2 1
#> [4,] 0 1 2
#> [5,] 0 3 0
#> [6,] 1 0 2
#> [7,] 0 2 1
#> [8,] 0 1 2
#> [9,] 0 2 1
#> [10,] 0 1 2
#>
# TODO: Needs fixing to support multiple inputs
# density(dist, 2)
# density(dist, 2, log = TRUE)
```