4.1.2.4 Atoms

An atom is identified by its name, which is a sequence characters, and can be written in any of the following forms:

• Any sequence of alphanumeric characters (including ‘_’), starting with a lowercase letter. Note that an atom may not begin with an underscore. The characters that are allowed to occur in such an unquoted atom are restricted to a subset of Unicode; see ref-syn-syn-tok.
• Any sequence from the following set of characters (except ‘/*’, which begins a comment):

  + - * / \ ^ < > = ~ : . ? @ # $ &
  

• Any sequence of characters delimited by single quotes. Backslashes in the sequence denote escape sequences (see ref-syn-syn-esc), and if the single quote character is included in the sequence, then it must be escaped, e.g. 'can\'t'. The characters that are allowed to occur in such a quoted atom are restricted to a subset of Unicode; see ref-syn-syn-tok.
• Any of:

  ! ; [] {}
  

  Note that the bracket pairs are special: ‘[]’ and ‘{}’ are atoms but ‘[’, ‘]’, ‘{’, and ‘}’ are not. The form [X] is a special notation for lists (see ref-syn-cpt-lis) as an alternative to .(X,[]), and the form {X} is allowed as an alternative to {}(X).

Examples of atoms are:

a   void   =   :=   'Anything in quotes'   []

Please note: It is recommended that you do not invent atoms beginning with the character ‘$’, since it is possible that such names may conflict with the names of atoms having special significance for certain built-in predicates.