Give prove, stepInto and columnItems a case per node kind, panicking on the rest

node.go

@@ -11,8 +11,9 @@ import (
 
 // node is a compiled element of the namespace tree: a folder, choice, null,
 // template, table, column or row. Compiling JSON into these once (see compile)
-// means rendering never re-inspects the raw JSON. A new kind needs a
-// case in contained, paths, render and renderEdges; each panics on a kind it does not list.
+// means rendering never re-inspects the raw JSON. A new kind needs a case in
+// columnItems, contained, paths, prove, render, renderEdges and stepInto; each
+// panics on a kind it does not list.
 type node interface{ isNode() }
 
 // folder is a namespace of named children, built from a directory of JSON files