Support the 'a' keyword in SPARQL lexer

lib/rouge/lexers/sparql.rb

@@ -35,20 +35,21 @@ def self.keywords
       state :root do
         rule %r(\s+)m, Text::Whitespace
         rule %r(#.*), Comment::Single
-        
+
         rule %r("""), Str::Double, :string_double_literal
         rule %r("), Str::Double, :string_double
         rule %r('''), Str::Single, :string_single_literal
         rule %r('), Str::Single, :string_single
-       
+
         rule %r([$?]\w+), Name::Variable
         rule %r((\w*:)(\w+)?) do |m|
           token Name::Namespace, m[1]
           token Str::Symbol, m[2]
         end
         rule %r(<[^>]*>), Name::Namespace
         rule %r(true|false)i, Keyword::Constant
-
+        rule %r/a\b/, Keyword
+
         rule %r([A-Z]\w+\b)i do |m|
           if self.class.builtins.include? m[0].upcase
             token Name::Builtin
@@ -101,7 +102,7 @@ def self.keywords
         end
         mixin :string_single_common
       end
-      
+
       state :string_single_literal do
         rule %r(''') do
           token Str::Single

spec/visual/samples/sparql

@@ -52,3 +52,11 @@ false
 '''The librarian said, "Perhaps you would enjoy 'War and Peace'."'''
 """The librarian said, 'Perhaps you would enjoy "War and Peace".'"""
 "\u4444 and \U88888888"
+
+# `a` keyword
+PREFIX : <http://example.org/#>
+SELECT ?item ?itemLabel
+WHERE
+{
+    ?item a :Test .
+}