docs/04-reference-dot.markdown @ 884333cfb6fb v1.3.2

Detect *all* cycles during topological sort

Previously the cycle detection was limited to detecting when we hit
a currently-being-visited node during a traversal.  So something like this would
be correctly found:

    A --> B --> C
          ^     |
          |     |
          +-----+

We start at the root (A), go to B, then to C, then to B, and detect that we're
still working on B and signal the error.

But this doesn't find all cycles, because we *start* at the root nodes, and if
a cycle doesn't have any outcropping branches we'll never reach it at all.  For
example:

    A --> B
    ^     |
    |     |
    +-----+

This graph has no roots, so we incorrectly ignore the cycle.

This patch fixes the problem by keeping a count of visited nodes and and making
sure it matches the digraph's size at the end.

Fixes https://github.com/sjl/cl-digraph/issues/4

# cl-dot Support

cl-digraph includes support for drawing digraphs with Graphviz using `cl-dot`
  in the `cl-digraphs.dot` system.

  [TOC]

## Package `DIGRAPH.DOT`

### `DRAW` (function)

    (DRAW DIGRAPH &KEY (FILENAME digraph.png) (FORMAT :PNG))

Draw `digraph` with cl-dot.