: " < > = == . ' % #We have already seen the use of the first seven of these. Single quotes can be used to form atoms that would otherwise be ill-formed as such; % is used for end-of-line comments, following the Prolog convention;
#is used to introduce declarations and other compiler directives (see Section 3.5, below).
The other classes, nodes, atoms and variables, must be
distinct, and distinct from the reserved symbols, but are otherwise
arbitrary (ormally, we require them to be finite classes, but
this is not of great significance here). For this discussion, we have
already adopted the convention that both nodes and atoms are simple
words, with nodes starting with uppercase letters. We extend this
convention to variables, discussed more fully in Section
3.4 below, which we require to start with the character
$. And we take white-space (spaces, newlines, tabs, etc.) to
delimit lexical tokens but otherwise to be insignificant.