Please replace raising an exception in the case of a cyclic graph with a non-exception-based return method, e.g. by returning `#f`.

The rationale behind this is that exceptions should be raised in error situations (violations, in terms of R6RS, in case of programmer errors, and errors in case of user/environment errors) and not for conveying a different-from-normal return value.

As using the procedure exported by SRFI 234 to test for cyclicity of a graph makes sense (especially when a topological sort is needed in case of non-cyclicity), in general situations, it is neither a programmer nor environment error when `topological-sort` is given a possibly cyclic graph.

Using the `#f` value for a cyclic graph, one can easily write

`(assert (topological-sort ...))`

instead of

`(topological-sort ...)`

to convey the fact that in this particular situation a non-cyclic graph is expected.

Marc