Guile has a Scheme level variable *features* that keeps track to
some extent of the features that are available in a running Guile.
*features* is a list of symbols, for example threads, each
of which describes a feature of the running Guile process.
A list of symbols describing available features of the Guile process.
You shouldn’t modify the *features* variable directly using
set!. Instead, see the procedures that are provided for this
purpose in the following subsection.