Comment refers to a person that knows bob and code would return a name, but this would only work if the triple
`person foaf:name bob .`
is part of the dataset. As this is a very uncommon way to model a `foaf:knows` the code was adjusted to match the description.

The code in docs won't always be complete programs and is sometimes just
snippets, and for snippets it does not make sense to error out if the
variables used in the snippet is not defined.