4.1.2.4 Atoms

An atom is identified by its name, which is a sequence of up to 65535 characters (other than the null character). An atom 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 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 it must be escaped, e.g. 'can\'t'.

  The characters that are allowed to occur in an 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'   []

WARNING: 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.