Providers can use this function to add proposals to the completion.
They can do so asynchronously by means of the finished argument. Providers must ensure that they always call this function
with finished set to true once each population (even if no proposals need to be added). Population occurs when
the populate function is called.
| this | |
| provider | |
| proposals |
The list of proposals to add. |
| finished |
Whether the provider is finished adding proposals. |