utility takes a list of pairs of node names representing directed arcs in
a graph and prints the nodes in topological order on standard output.
Input is taken from the named
or from standard input if no file
There must be an even number of nodes in the input.
Node names specified on the same line should be white space separated.
Presence of a node in a graph can be represented by an arc from the node
This is useful when a node is not connected to any other nodes.
If the graph contains a cycle (and therefore cannot be properly sorted),
one of the arcs in the cycle is ignored and the sort continues.
Cycles are reported on standard error.
The options are as follows:
Turn on debugging.
Search for and display the longest cycle.
Can take a very long time.
Do not display informational messages about cycles.
This is primarily
intended for building libraries, where optimal ordering is not critical,
and cycles occur often.