Tested each sample.

gem/ex3.lua

@@ -0,0 +1,15 @@
+local function chainpair(f1, f2)
+  return function(chunk)
+    local ret = f2(f1(chunk))
+    if chunk then return ret
+    else return (ret or "") .. (f2() or "") end
+  end
+end
+
+function filter.chain(...)
+  local f = select(1, ...) 
+  for i = 2, select('#', ...) do
+    f = chainpair(f, select(i, ...))
+  end
+  return f
+end