If set_source_model has been used to provide a hint that
this's value should be among the values contained in a column of a data model, then this method returns
which data model, and if col is not null, then it is set to the restricting column as well.
Otherwise, this method returns null, and if col is not null, then it is set to 0.
| this |
a Holder |
| col |
a place to store the column in the model sourcing the holder, or |
|
a pointer to a DataModel, or |