An S4 class to represent a stratigraphic matrix.

A stratigraphic matrix represents directed relationships between stratigraphic units. A stratigraphic matrix is an adjacency matrix (a non symmetric square matrix with zeros on its main diagonal), suitable to build a directed acyclic graph (DAG).

N. Frerebeau

```
# Principles of Archaeological Stratigraphy, fig. 12
harris <- matrix(
data = c(2, 1,
3, 1,
4, 1,
5, 2,
5, 3,
5, 4,
6, 5,
7, 1,
7, 6,
8, 1,
8, 6,
9, 7,
9, 8),
ncol = 2,
byrow = TRUE,
dimnames = list(NULL, c("lower", "upper"))
)
strati <- as_stratigraphy(harris)
```