Numeric variables or polynomial terms that involve them can be included in the model as regression effects (covariates). The
actual values of such terms are included as columns of the relevant model matrices. You can use the bar operator
along with a regression effect to generate polynomial effects. For example, `X`

| `X`

| `X`

expands to `X`

`X`

*`X`

`X`

*`X`

*`X`

, which is a cubic model.