extra/central implements the "central" pattern

diff --git a/extra/central/authors.txt b/extra/central/authors.txt
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+Matthew Willis

diff --git a/extra/central/central-docs.factor b/extra/central/central-docs.factor
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+USING: central help.markup help.syntax ;
+
+HELP: CENTRAL:
+{ $description
+    "This parsing word defines a pair of words useful for "
+    "implementing the \"central\" pattern: " { $snippet "symbol" } " and "
+    { $snippet "with-symbol" } ".  This is a middle ground between excessive "
+    "stack manipulation and full-out locals, meant to solve the case where "
+    "one object is operated on by several related words."
+} ;
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diff --git a/extra/central/central-tests.factor b/extra/central/central-tests.factor
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+USING: central tools.test ;
+
+IN: scratchpad
+
+CENTRAL: test-central
+
+[ 3 ] [ 3 [ test-central ] with-test-central ] unit-test
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diff --git a/extra/central/central.factor b/extra/central/central.factor
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+USING: kernel lexer namespaces parser sequences words ;
+
+IN: central
+
+: define-central-getter ( word -- )
+    dup [ get ] curry (( -- obj )) define-declared ;
+
+: define-central-setter ( word with-word -- )
+    [ with-variable ] with (( object quot -- )) define-declared ;
+
+: define-central ( word-name -- )
+    [ create-in dup define-central-getter ] keep
+    "with-" prepend create-in [ define-central-setter ] keep
+    make-inline ;
+
+SYNTAX: CENTRAL: ( -- ) scan define-central ;
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diff --git a/extra/central/tags.txt b/extra/central/tags.txt
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+extensions