/usr/local/lib/swipl/library/prolog_colour.pl
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 syntax_colour(?Class, ?Attributes) is nondet
True when a range classified Class must be coloured using Attributes. Attributes is a list of:
  • colour(ColourName)
  • background(ColourName)
  • bold(Boolean)
  • underline(Boolean)

Attributes may be the empty list. This is used for cases where -for example- a menu is associated with the fragment. If syntax_colour/2 fails, no fragment is created for the region.