Atoms

An atom is:

• Anything enclosed in single quotes (e.g. 'Whatever you like').
• Any sequence of characters, numbers and/or the underscore character which is preceded by a lower case character (e.g. this_IS_an_atom).
• Any continuous sequence of the symbols: +, -, *, /, \, ^, >, <, =, ', :, ., ?, @, #, $, &. (e.g. ***+***+@).
• Any of the special atoms: [], {}, ;, !.

The Prolog built-in predicate atom/1 will tell you if a given term is or isn't an atom. It gives the following behaviour:

| ?- atom(foo).
yes
| ?- atom(Foo).
no
| ?- atom('Foo').
yes
| ?- atom([]).
yes