`is_dag()`

checks if a graph has a topological ordering (i.e. is a directed
acyclic graph) using Kahn's algorithm.

## Arguments

- x
An adjacency

`matrix`

to be tested.

## Value

A `logical`

scalar.

## References

Kahn, A. B. (1962). Topological sorting of large networks. *Communications
of the ACM*, 5(11), p. 558-562. doi:10.1145/368996.369025
.