diff --git a/builder.nix b/builder.nix
index f21961b..d98ff37 100644
--- a/builder.nix
+++ b/builder.nix
@@ -20,7 +20,7 @@ let
       funcPkgs = {
         __functor = self: arg: resolve arg;
         src = getSrc root rcwd filter;
-        inherit pkgs cwd rcwd;
+        inherit pkgs cwd rcwd d1 d2;
       };
 
       _builder = let
diff --git a/lib.nix b/lib.nix
index c32aef8..776e821 100644
--- a/lib.nix
+++ b/lib.nix
@@ -1,10 +1,10 @@
 { lib ? import <nixpkgs/lib> }:
 let
   inherit (builtins) baseNameOf dirOf length genList pathExists filterSource
-  isString match replaceStrings;
+  isString match replaceStrings stringLength;
   inherit (lib) take splitString concatStringsSep last foldl foldr head tail
   singleton removePrefix hasSuffix removeSuffix flatten crossLists reverseList
-  all any unique;
+  all any unique hasPrefix;
   butlast = list: take (length list - 1) list;
 
   doFileSuffix = "do";
@@ -77,7 +77,7 @@ let
   # Add all preceding paths to a src down to the root. Otherwise
   # builtins.filterSource will reject it.
   augmentedSrcs = root: srcs: let
-    aug = map (s: singleton s ++ (map (p: root + (removeSuffix "/" p)) (genPathlist (removePrefix root s)))) srcs;
+    aug = map (s: (map (p: root + (removeSuffix "/" p)) (genPathlist (removePrefix root s)))) srcs;
   in unique (flatten aug);
 
   # The source tree resolver combinator.
@@ -90,15 +90,25 @@ let
     captureFunc (resolveSrc root rcwd) (srcs: let
 
       rejector = path: (all (x: path != x) rejected);
+
+      # If a src in srcs is a prefix to a that (it must be a dir, then, or the
+      # file itself), allow all sub-paths.
+      srcFilter = path:
+        if srcs == []
+        then true
+        else (any (x: (hasPrefix x path)) srcs);
+
+      # Allow all directories lying on the way to specified srcs.
       _srcs = (augmentedSrcs root srcs);
-      srcFilter = path: if srcs == []
+      srcDirFilter = path: if srcs == []
                         then true # No explicit srcs means: Take the whole source tree!
-                        else (any (x: path == x) _srcs);
+                        else (any (x: path == x) (_srcs));
       filter = (path: type:
         (baseNameOf path != ".git") &&
         (baseNameOf path != ".envrc") &&
         (rejector path) &&
-        (srcFilter path));
+        ((srcFilter path) ||
+         (srcDirFilter path)));
     in (builtins.filterSource filter root));
 
   self = {