An S4 class to represent a stratigraphic matrix.

## Details

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).

## Note

This class inherits from AdjacencyMatrix.

## Examples

```
## Principles of Archaeological Stratigraphy, fig. 12
## Build an adjacency list
harris <- data.frame(
younger = c(1, 1, 1, 2, 3, 4, 5, 6, 6, 6, 7, 8, 1, 1),
older = c(2, 3, 4, 5, 5, 5, 6, 7, 8, 9, 9, 9, 7, 8)
)
## Coerce to adjacency matrix
strati <- as_stratigraphy(harris, reduce = TRUE)
## Define interface layers
set_interface(strati) <- c("1" = FALSE, "2" = TRUE, "3" = FALSE,
"4" = FALSE, "5" = FALSE, "6" = TRUE,
"7" = FALSE, "8" = FALSE, "9" = FALSE)
## Stratigraphic graph
graph <- as_graph(strati)
plot(graph)
```