+ * 'middle' and 'last' are the types of the middle
+ and last nodes of the graph over which the dataflow
+ solution is being computed
+
+ * 'input' is an input, from which it should be possible to
+ derive a dataflow fact of interest. For example, 'input' might
+ be equal to a fact, or it might be a tuple of which one element
+ is a fact.
+
+ * 'output' is an output, or possibly a function from 'fuel' to an
+ output
+
+A computation is interesting for any pair of 'middle' and 'last' type
+parameters that can form a reasonable graph. But it is not useful to
+instantiate 'input' and 'output' arbitrarily. Rather, only certain
+combinations of instances are likely to be useful, such as those shown
+below.
+
+Backward analyses compute *in* facts (facts on inedges).