Dependencies are other reactive variables.

Examples:

```
import asyncio
import time
import solara
from solara.lab import reactive_task

x = solara.reactive(2)

@reactive_task
async def x_square():
    await asyncio.sleep(2)
    a = b
    return x.value**2

@solara.component
def Page():
    solara.SliderInt("x", value=x, min=0, max=10)
    if x_square.value.state == solara.ResultState.FINISHED:
        solara.Text(repr(x_square.value.value))
    solara.ProgressLinear(x_square.value.state == solara.ResultState.RUNNING)
```