begin gm docs

docs/src/commentary/gm.rst

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+The *G-Machine*
+===============
+
+**********
+Motivation
+**********
+
+Our initial model, the *Template Instantiation Machine* (TIM) was a very
+straightforward solution to compilation, but its core design has a major
+Achilles' heel, being that Compilation is interleaved with evaluation -- The
+heap nodes for supercombinators hold uninstantiated expressions, i.e. raw ASTs
+straight from the parser. When a supercombinator is found on the stack during
+evaluation, the template expression is instantiated (compiled) on the spot.
+
+.. math::
+   \transrule
+   { a_0 : a_1 : \ldots : a_n : s
+   & d
+   & h
+   \begin{bmatrix}
+        a_0 : \mathtt{NSupercomb} \; [x_1,\ldots,x_n] \; e
+   \end{bmatrix}
+   & g
+   }
+   { a_n : s
+   & d
+   & h'
+   & g
+   \\
+   & \SetCell[c=3]{c}
+   \text{where } h' = \mathtt{instantiateU} \; e \; a_n \; h \; g
+   }
+
+The process of instantiating a supercombinator goes something like this
+
+1. Augment the environment with bindings to the arguments.
+
+2. Using the local augmented environment, instantiate the supercombinator body
+   on the heap.
+
+3. Remove the nodes applying the supercombinator to its arguments from the
+   stack.
+
+4. Push the address to the newly instantiated body onto the stack.
+
+.. literalinclude:: /../../src/TIM.hs
+   :dedent:
+   :start-after: -- >> [ref/scStep]
+   :end-before: -- << [ref/scStep]
+   :caption: src/TIM.hs
+
+Instantiating the supercombinator's body in this way is the root of our
+Achilles' heel. Traversing a tree structure is a very non-linear task unfit for
+an assembly target. The goal of our new G-Machine is to compile a *linear
+sequence of instructions* which instantiate the expression at execution.
+
+**************
+Implementation
+**************
+
+
+