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diff --git a/core/base/action.moon b/core/base/action.moon
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+----
+-- Builtin / Special Form / `Cell`-evaluation Strategy.
+--
+-- Responsible for quoting/evaluating subexpressions, instantiating and patching
+-- `Op`s, updating the current `Scope`, etc.
+-- See `builtin` and `invoke` for examples.
+--
+-- @classmod Action
+
+class Action
+--- Action interface.
+--
+-- methods that have to be implemented by `Action` implementations.
+-- @section interface
+
+  --- create a new instance
+  -- @tparam Value head the (`\eval`d) head of the Cell to evaluate
+  -- @tparam Tag tag the Tag of the expression to evaluate
+  new: (head, @tag) =>
+    @patch head
+
+  --- perform the actual evaluation
+  --
+  -- Implementations should:
+  --
+  -- - eval or quote `tail` values
+  -- - perform scope effects
+  -- - wrap all child-results
+  --
+  -- @tparam Scope scope the active scope
+  -- @tparam {AST,...} tail the arguments to this expression
+  -- @treturn Result the result of this evaluation
+  eval: (scope, tail) => error "not implemented"
+
+  --- free resources
+  destroy: =>
+
+  --- attempt to update this instance with a new `@head` prior to `\eval`.
+  --
+  -- If `\patch` returns `false`, this instance is `\destroy`ed and recreated.
+  -- Must *not* return `false` when called immediately after `\new`.
+  -- Only considered if Action types of old and new expression match.
+  --
+  -- @tparam AST head the new head value
+  -- @treturn bool whether patching was successful
+  patch: (head) =>
+    if head == @head
+      true
+
+    @head = head
+
+--- static functions
+-- @section static
+
+  --- get-or-update an `Action` for a given tag, then evaluate it.
+  --
+  -- Find the action for the expression with `Tag` `tag` if it exists,
+  -- and is compatible with the new `head`, otherwise instantiate one.
+  -- Register the `Action` with `tag`, evaluate it and return the `Result`.
+  --
+  -- @tparam Scope scope the active scope
+  -- @tparam Tag tag the tag of the `Cell` being evaluated
+  -- @tparam Value head the (`\eval`d) head of the `Cell` being evaluated
+  -- @tparam {Ast,...} tail the raw AST parameters to the `Cell` being evaluated
+  -- @treturn Result the result of evaluation
+  eval_cell: (scope, tag, head, tail) =>
+    last = tag\last!
+    compatible = last and
+                 (last.__class == @) and
+                 (last\patch head) and
+                 last
+
+    L\trace if compatible
+      "reusing #{last} for #{tag} <#{@__name} #{head}>"
+    else if last
+      "replacing #{last} with new #{tag} <#{@__name} #{head}>"
+    else
+      "initializing #{tag} <#{@__name} #{head}>"
+
+    action = if compatible
+      tag\keep compatible
+      compatible
+    else
+      last\destroy! if last
+      with next = @ head, tag
+        tag\replace next
+
+    action\eval scope, tail
+
+  __tostring: => "<#{@@__name} #{@head}>"
+  __inherited: (cls) => cls.__base.__tostring = @__tostring
+
+{
+  :Action
+}