Finite Domains: fd

### Finite Domains: `fd`

Field | Mutable | Type | Explanation
--- | --- | --- | ---
`variables` | no | `Int` | Number of finite domain variables created.
`propagators` | no | `Int` | Number of finite domain propagators created.
`invoked` | no | `Int` | Number of finite domain propagators invoked.
`threshold` | yes | `Int` | Integer when internal domain representation switches from bit sets to interval lists.