SWI-Prolog -- Manual

Documentation
- Reference manual
  - Overview
    - The SWI-Prolog syntax
      - ISO Syntax Support
        
        Processor Character Set
        
        Nested comments
        
        Character Escape Syntax
        
        Syntax for non-decimal numbers
        
        Using digit groups in large integers
        
        Rational number syntax
        
        NaN and Infinity floats and their syntax
        
        Force only underscore to introduce a variable
        
        Unicode Prolog source
        
        Singleton variable checking
- Packages

2.16.1.1 Processor Character Set

The processor character set specifies the class of each character used for parsing Prolog source text. Character classification is fixed to Unicode. See also section 2.19.