Suppose $S=\set{B_1,\,B_2,\,B_3,\,\ldots,\,B_p}$ is a set of matrices from $M_{mn}$. Formulate appropriate definitions for the following terms and give an example of the use of each.

1. A linear combination of elements of $S$.
2. A relation of linear dependence on $S$, both trivial and nontrivial.
3. $S$ is a linearly independent set.
4. $\spn{S}$.