Market model formula

## Usage

```
# S4 method for market_model
formula(x)
```

## Details

Market model formulas adhere to the following specification:

`quantity | price | subject | time ~ demand | supply`

where

quantity: The model's traded (observed) quantity variable.

price: The model's price variable.

quantity: The model's subject (e.g. firm) identification variable.

quantity: The model's time identification variable.

demand: The right hand side of the model's demand equation.

supply: The right hand side of the model's supply equation.

The `diseq_stochastic_adjustment`

additionally specify
price dynamics by appending the right hand side of the equation at the end
of the formula, i.e.

`quantity | price | subject | time ~ demand | supply | price_dynamics`

The left hand side part of the model formula specifies the elements that are needed for initializing the model. The market models of the package prepare the data based on these four variables using their respective identification assumptions. See market model classes for more details.

The function provides access to the formula used in model initialization.

## Examples

```
# \donttest{
model <- simulate_model(
"diseq_stochastic_adjustment", list(
# observed entities, observed time points
nobs = 500, tobs = 3,
# demand coefficients
alpha_d = -0.1, beta_d0 = 9.8, beta_d = c(0.3, -0.2), eta_d = c(0.6, -0.1),
# supply coefficients
alpha_s = 0.1, beta_s0 = 6.1, beta_s = c(0.9), eta_s = c(-0.5, 0.2),
# price equation coefficients
gamma = 1.2, beta_p0 = 3.1, beta_p = c(0.8)
),
seed = 31
)
# access the model's formula
formula(model)
#> Q | P | id | date ~ P + Xd1 + Xd2 + X1 + X2 | P + Xs1 + X1 +
#> X2 | Xp1
#> <environment: 0x56123f659390>
# }
```