@@ -86,8 +86,11 @@
 	ext = ss.exnkind 'extension error';
 	enc = ss.exnkind('encoding error', function(msg, ...)
 		return string.format('[%s]' .. msg, ...)
 	end);
+	rdr = ss.exnkind('could not render', function(msg, ...)
+		return string.format('(backend %s)'..msg, ...)
+	end);
 }
 
 ct.ctx = declare {
 	mk = function(src) return {src = src} end;
@@ -144,8 +147,25 @@
 	construct = function(self, id, depth)
 		self.id = id
 		self.depth = depth
 	end;
+	fns = {
+		visible = function(self)
+			if self.kind == 'nonprinting' then return false end
+			local invisibles = {
+				['break'] = true;
+				reference = true;
+				resource = true;
+				directive = true;
+			}
+			for k,b in pairs(self.blocks) do
+				if not (invisibles[b.kind] or b.invisible) then return true end
+				-- extensions that add invisible nodes to the AST must
+				-- mark them as such for rendering to work properly!
+			end
+			return false
+		end;
+	}
 }
 
 ct.doc = declare {
 	ident = 'doc';
@@ -752,9 +772,9 @@
 					break
 				end
 			end
 			if not found then
-				ctx:fail('no recognized control sequence in [%s]', substr)
+				buf = buf .. c
 			end
 		elseif c == '\n' then
 			flush()
 			table.insert(spans,{kind='line-break',origin=ctx:clone()})
@@ -989,9 +1009,11 @@
 		local str = l:match '^\t\t(.-)%s*$'
 		last.val = last.val .. '\n' .. str
 		c.sec.refs[last.key] = last.val
 	end};
-	{seq = '\t', fn = blockwrap(function(l,c,j,d)
+	{seq = '\t', pred = function(l)
+		return (l:match '\t+([^:]+):%s*(.*)$')
+	end; fn = blockwrap(function(l,c,j,d)
 		local ref, val = l:match '\t+([^:]+):%s*(.*)$'
 		local last = d[#d]
 		local rsrc
 		if last and last.kind == 'resource' then
@@ -1188,9 +1210,11 @@
 	job:hook('line_end',ctx,l)
 end
 
 function ct.parse(file, src, mode, setup)
-
+	-- this object is threaded down through the parse tree
+	-- and copied to store information like the origin of the
+	-- element in the source code
 	local ctx = ct.ctx.mk(src)
 	ctx.line = 0
 	ctx.doc = ct.doc.mk()
 	ctx.doc.src = src
@@ -1270,4 +1294,23 @@
 	ctx.doc.stage = nil
 	ctx.doc.docjob:hook('meddle_ast')
 	return ctx.doc
 end
+
+function ct.expand_var(v)
+	local val
+	if v.pos then
+		if not v.origin.invocation then
+			v.origin:fail 'positional arguments can only be used in a macro invocation'
+		elseif not v.origin.invocation.args[v.pos] then
+			v.origin.invocation.origin:fail('macro invocation %s missing positional argument #%u', v.origin.invocation.macro, v.pos)
+		end
+		val = v.origin.invocation.args[v.pos]
+	else
+		val = v.origin.doc:context_var(v.var, v.origin)
+	end
+	if v.raw then
+		return val, true
+	else
+		return ct.parse_span(val, v.origin), false
+	end
+end