type _type
= [
| `button |
| `checkbox |
| `color |
| `date |
| `datetime_local |
| `email |
| `file |
| `hidden |
| `image |
| `month |
| `number |
| `password |
| `radio |
| `range |
| `reset |
| `search |
| `submit |
| `tel |
| `text |
| `time |
| `url |
| `week |
]
type step
= [
| `any |
| `step_value of float |
]