Minimal Cover for a Set of FDs
A minimal cover for a set F of FDs is a set G of FDs such that:
 1. Every dependency in G is of the form X → A, where A is a single attribute.
 2. The closure F + is equal to the closure G+ .
 3. If we obtain a set H of dependencies from G by deleting one or more dependencies,
     or by deleting attributes from a dependency in G, then F + = H + .
Intuitively, a minimal cover for a set F of FDs is an equivalent set of dependencies that
is minimal in two respects: (1) Every dependency is as small as possible; that is, each
attribute on the left side is necessary and the right side is a single attribute. (2) Every
dependency in it is required in order for the closure to be equal to F + .