Embedding Pubmed, Graphviz and a remote image in #LaTeX by Pierre Lindenbaum.
Pierre demonstrates how to use:
\newcommand{name}[num]{definition}
to load a remote picture, a graphviz result and retrieving a PubMed record for embedding in a LaTeX document.
From the LaTeX Macro page Pierre cites:
The num argument in square brackets is optional and specifies the number of arguments the new command takes (up to 9 are possible). If missing it defaults to 0, i.e. no argument allowed.
I caught myself wondering about that argument.
The graphviz command looks particularly interesting for topic map illustrations.