4.1.3 Atoms

A HiLog atom is identified by its name, which is a sequence of up to 1000 characters (other than the null character). Just like a Prolog atom, a HiLog atom can be written in any of the following forms:

• Any sequence of alphanumeric characters (including '_'), starting with a lowercase letter.

• Any sequence from the following set of characters (except of the sequence '/*', which begins a comment):

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

• Any sequence of characters delimited by single quotes, such as:

                           'sofaki'    '%'    '_$op'
        

  If the single quote character is to be included in the sequence it must be written twice. For example:

                               'don''t'      ''''
  

• Any of the following:

                                   !  ;  []  {}
  

  Note that the bracket pairs are special. While '[]' and '$\{\}$' are atoms, '[', ']', '$\{$', and '$\}$' are not. Like Prolog, the form [X] is a special notation for lists (see Section 4.1.6), while the form $\{$X$\}$ is just ``syntactic sugar'' for the term '$\{\}$'(X).

Examples of HiLog atoms are:

               h   foo   ^=..   ::=   'I am also a HiLog atom'   []