A functional dependency $F: X \rightarrow Y$ is termed as a useful functional dependency if and only if it satisfies all the following three conditions:
- $\text{X}$ is not the empty set.
- $\text{Y}$ is not the empty set.
- Intersection of $\text{X}$ and $\text{Y}$ is the empty set.
For a relation $\text{R}$ with $4$ attributes, the total number of possible useful functional dependencies is __________.
