diff --git a/core/src/file.sld b/core/src/file.sld
index 7212c73..0a83c80 100644
--- a/core/src/file.sld
+++ b/core/src/file.sld
@@ -5,7 +5,8 @@
     (zilch magic) (zilch nix binproto) (zilch nix daemon) (zilch nix drv) (zilch zexpr)
     (chicken base) (chicken format)
     (srfi 128) (srfi 132) (srfi 146) (srfi 151))
-  (export zfile zsymlink zdir 
+  (export zfile zsymlink zdir
+          z-file? z-directory? z-symlink?
           zfile->store)
 
   (begin
diff --git a/lang/ninja/src/nixpkgs.sld b/lang/ninja/src/nixpkgs.sld
index fb28516..5032a2a 100644
--- a/lang/ninja/src/nixpkgs.sld
+++ b/lang/ninja/src/nixpkgs.sld
@@ -5,13 +5,15 @@
    (zilch lang ninja) (zilch lang ninja build)
    (zilch lang ninja config)
    (zilch magic) (zilch nixpkgs) (zilch vfs)
+   (zilch file)
    (zilch nix drv)
    (zilch zexpr)
-   (srfi 132) (srfi 146) (srfi 152))
+   (srfi 128) (srfi 132) (srfi 146) (srfi 152))
 
  (export
   setup-ninja-environment
-  build-nixpkgs-drv-reproducibly)
+  build-nixpkgs-drv-reproducibly
+  determine-data-flow)
 
  (begin
   (define coreutils (cdr (assoc "out" (nixpkgs "coreutils"))))
@@ -233,4 +235,119 @@
        `(("buildCommand" . ,postbuild-builder))
        placeholders)
       (map car placeholders))
-     export-depfile))))
+     export-depfile))
+  
+  ;; Build the derivation, but with stubbed out header and .so files.
+  ;; This is used to determine the dataflow, to make cross-project incremental
+  ;; builds work.
+  ;; Returns a single SRFI-146 mapping, containing keys of shape `(output . (dir . name))`,
+  ;; and values either a zexpr-y store path, or a pair `(marker . <name of the build output>)`
+  (define (determine-data-flow conf)
+    (define-values (initial-drv configured-drv placeholders edge-ref defaults export-depfile) (setup-ninja-environment conf))
+
+    (define edges (built-edge-phony-inputs (cdr (edge-ref "all"))))
+
+    (define fix-placeholders-command "")
+    (define copy-in-place-command "")
+
+    ; Append the necessary commands to fix up the fake store paths post-install but pre-everything-else.
+    (for-each
+     (lambda (plc)
+       (set! fix-placeholders-command
+         (string-append fix-placeholders-command
+          "zilchFixPlaceholder \"" (cdr plc) "\" \"$" (car plc) "\"\n"))
+       (set! copy-in-place-command
+         (string-append copy-in-place-command
+          "cp -rf ../out" (cdr plc) " \"$" (car plc) "\" || mkdir \"$" (car plc) "\"\n")))
+     placeholders)
+
+    ; Turn the original `src` and `build` into a known store path.
+    (define realised-store (store-path-devirtualise configured-drv))
+
+    (define make-all-placeholder-files
+     (string-join (map (lambda (v) (string-append "zilchMakeFile \"" v "\"\n")) edges) "\n"))
+
+    ; Prepare the post-build builder. This puts everything in its place and runs the post-build phases from the original .drv.
+    (define postbuild-builder
+      #~,(string-append
+          "zilchMakeFile() {\n"
+           "mkdir -p bdir/build/$(dirname \"$1\")\n"
+           "rm bdir/build/\"$1\" || true\n"
+           "echo \"ZILCH MARKER FILE ->$1\" > bdir/build/\"$1\"\n"
+          "}\n"
+          "zilchPlace() {\n"
+           "cd $NIX_BUILD_TOP; cp -rf --no-preserve=ownership " #$realised-store " bdir\n"
+           "chmod ugo+rw -R bdir\n"
+           "(cd " #$realised-store "/src; find . -type f '(' -name '*.h' -o -name '*.hh' -o -name '*.so' ')') | while read f; do zilchMakeFile \"../src/$f\"; done\n"
+           make-all-placeholder-files
+           "cd bdir/build\n"
+          "}\n"
+          "mesonBuildDir=$NIX_BUILD_TOP/bdir/build cmakeBuildDir=$NIX_BUILD_TOP/bdir/build cmakeDir=\"$NIX_BUILD_TOP/bdir/src/${cmakeDir:-.}\"\n"
+          "zilchFixPlaceholder() {\n"
+          " find ../out -type f -exec sed -i -e \"s|$1|$2|g\" \"{}\" \";\" || exit 1\n"
+          " find \"../out\" -type l | while read link; do\n"
+          "  target=\"$(readlink \"$link\")\"; rewritten=\"$(printf \"%s\" \"$target\" | sed -e \"s|$1|$2|g\")\"\n"
+          "  rm \"$link\" && ln -s \"$rewritten\" \"$link\" || exit 1\n"
+          " done\n"
+          "}\n"
+          "zilchFixup() {\n"
+          fix-placeholders-command
+          copy-in-place-command
+          "}\n"
+          "zilchMesonInstall() {\n"
+          " runHook preInstall\n"
+          " local flagsArray=()\n"
+          " if [[ -n \"$mesonInstallTags\" ]]; then\n"
+          "  flagsArray+=(\"--tags\" \"$(concatStringsSep \",\" mesonInstallTags)\")\n"
+          " fi\n"
+          " concatTo flagsArray mesonInstallFlags mesonInstallFlagsArray\n"
+          " DESTDIR=$NIX_BUILD_TOP/bdir/out meson install --no-rebuild \"${flagsArray[@]}\"\n"
+          " zilchFixup\n"
+          " runHook postInstall\n"
+          "}\n"
+          "zilchCmakeInstall() {\n"
+          " runHook preInstall\n"
+          " DESTDIR=$NIX_BUILD_TOP/bdir/out cmake --install .\n"
+          " zilchFixup\n"
+          " runHook postInstall\n"
+          "}\n"
+          "if [[ $(type -t mesonInstallPhase) == function ]]; then\n"
+          "  installPhase=zilchMesonInstall; checkPhase=mesonCheckPhase\n"
+          "else\n"
+          "  installPhase=zilchCmakeInstall; checkPhase=ninjaCheckPhase\n"
+          "fi\n"
+          "phases=\"zilchPlace checkPhase ${preInstallPhases[*]:-} installPhase ${preFixupPhases[*]:-} fixupPhase installCheckPhase ${preDistPhases[*]:-} distPhase ${postPhases[*]:-}\"\n"
+          "for curPhase in ${phases[*]}; do runPhase \"$curPhase\"; done\n"))
+
+    ; Patch the original .drv to run the postbuild-builder command.
+    (define patched-drv
+     (patch-drv initial-drv
+      (append
+       `(("buildCommand" . ,postbuild-builder))
+       placeholders)
+      (map car placeholders)))
+    (define (get-file-marker fptr)
+      (call-with-port (store-path-open fptr)
+       (lambda (p)
+        (define header (read-string 20 p))
+        (and (string=? header "ZILCH MARKER FILE ->")
+         (let ((str (read-string 99999 p)))
+           (string-copy str 0 (- (string-length str) 1)))))))
+    (define output (mapping (make-default-comparator)))
+    (define (process-output name-data-pair)
+      (define name (car name-data-pair))
+      (define store-path (cdr name-data-pair))
+      (define vfs (vfs-from-store store-path))
+      (mapping-for-each
+       (lambda (path val)
+         ; TODO(puck): this depends on vfs-from-store internals
+        (define is-file (z-file? val))
+        (define marker (and is-file (get-file-marker val)))
+        (if marker
+          (set! output (mapping-set! output (cons name path) (cons 'marker marker)))
+          (set! output (mapping-set! output (cons name path) val))))
+       (vfs-contents vfs)))
+    (for-each process-output patched-drv)
+    output)))
+
+    ; TODO(puck): for each output, do the necessary dance of figuring out where it came from. read first N bytes, compare, then do the thing. output a big alist and do the dataflow dance?