util.toposort- Topological sort
Implements topological sort algorithm.
Graph represents a directed acyclic graph (DAG) by a list of connections, where each connection is the form
(<node> <downstream> <downstream2> ...)
that means a node
<node> is connected to other nodes
<node> can be arbitrary
object, as far as it can be compared by the procedure eqproc,
which is eqv? by default (see Equality).
Returns a list of
<node>s sorted topologically.
If the graph contains circular reference, an error is signaled.