add singleton classes

extra/singleton/authors.txt

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+Doug Coleman

extra/singleton/singleton-docs.factor

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+USING: help.markup help.syntax ;
+IN: singleton
+
+HELP: SINGLETON:
+{ $syntax "SINGLETON: class"
+} { $values
+    { "class" "a new tuple class to define" }
+} { $description
+    "Defines a new tuple class with membership predicate name? and a default empty constructor that is the class name itself."
+} { $examples
+    { $example "SINGLETON: foo\nfoo ." "T{ foo f }" }
+} { $see-also
+    POSTPONE: TUPLE:
+} ;

extra/singleton/singleton.factor

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+! Copyright (C) 2007 Doug Coleman.
+! See http://factorcode.org/license.txt for BSD license.
+USING: kernel parser quotations tuples words ;
+IN: singleton
+
+: SINGLETON:
+    CREATE-CLASS
+    dup { } define-tuple-class
+    dup construct-empty 1quotation define ; parsing