diff --git a/README.md b/README.md
index aacfdf1..03c932d 100644
--- a/README.md
+++ b/README.md
@@ -35,9 +35,7 @@ The following goes recursively through `./modules` and imports all `.nix` files.
 }
 ```
 
-For more advanced usage, `import-tree` can be configured via its builder API.
-This means that the result of calling a function on an `import-tree` object
-is itself another `import-tree` object.
+For more advanced usage, `import-tree` can be configured via its extensible API.
 
 </summary>
 
@@ -94,20 +92,28 @@ The following is valid usage:
 ```
 
 As an special case, when the single argument given to an `import-tree` object is an
-attribute-set *meaning it is _NOT_ a path or list of paths*, the `import-tree` object
-assumes it is being imported as a module. This way, a pre-configured `import-tree` can
+attribute-set *-it is _NOT_ a path or list of paths-*, the `import-tree` object
+assumes it is being evaluated as a module. This way, a pre-configured `import-tree` can
 also be used directly in a list of module imports.
 
 This is useful for authors exposing pre-configured `import-tree`s that users can directly
-add to their import list or continue configuring themselves using the API.
+add to their import list or continue configuring themselves using its API.
 
 ```nix
 let
-  # imagine this configured tree is actually provided by some flake or library.
-  # users can directly import it or continue using API methods on it.
-  configured-tree = import-tree.addPath [./a [./b]]; # paths are configured by library author.
+  # imagine this configured tree comes from some author's flake or library.
+  # library author can extend an import-tree with custom API methods
+  # according to the library's directory and file naming conventions.
+  configured-tree = import-tree.addAPI {
+    # the knowledge of where modules are located inside the library structure
+    # or which filters/regexes/transformations to apply are abstracted 
+    # from the user by the author providing a meaningful API.
+    maximal = self: self.addPath ./modules;
+    minimal = self: self.maximal.filtered (lib.hasInfix "minimal");
+  };
 in {
-  imports = [ configured-tree ]; # but then imported or further configured by the library user.
+  # the library user can directly import or further configure an import-tree.
+  imports = [ configured-tree.minimal ];
 }
 ```
 
@@ -221,6 +227,26 @@ This function can be applied multiple times.
 import-tree [./vendor ./modules]
 ```
 
+### `import-tree.addAPI`
+
+`addAPI` extends the current import-tree object with new methods.
+The API is cumulative, meaning that this function can be called multiple times.
+
+`addAPI` takes an attribute set of functions taking a single argument:
+`self` which is the current import-tree object.
+
+```nix
+# import-tree.addAPI : api-attr-set -> import-tree
+
+import-tree.addAPI {
+  maximal = self: self.addPath ./modules;
+  feature = self: featureName: self.maximal.filtered (lib.hasInfix feature);
+  minimal = self: self.feature "minimal";
+}
+```
+
+on the previous API, users can call `import-tree.feature "+vim"` or `import-tree.minimal`, etc.
+
 ### `import-tree.withLib`
 
 > \[!NOTE\]
@@ -283,26 +309,42 @@ to learn about the Dendritic pattern advantages.
 
 ### Sharing pre-configured subtrees of modules
 
-Since the import-tree API lets you prepend paths and filter which files to include,
-you could have flakes that output different sets of pre-configured trees.
+<details>
+<summary>
+
+Since the import-tree API is _extensible_ and lets you add paths or
+filters at configuration time, library authors can provide pre-configured
+import-trees and custom API methods for their particular needs.
+
+</summary>
 
 This would allow us to have community-driven *sets* of configurations,
 much like those popular for editors: spacemacs/lazy-vim distributions.
 
-Imagine an editor distribution exposing the following `lib.trees` flake output:
+Imagine an editor distribution exposing the following flake output:
 
 ```nix
 # editor-distro's flakeModule
 {inputs, lib, ...}:
 let 
-  flake.lib.trees = {
-    inherit root on off xor;
-  };
+  flake.lib.modules-tree = lib.pipe import-tree [
+    (i: i.addPath ./modules)
+    (i: i.addAPI { inherit hadDirMatching on off exclusive; })
+    (i: i.addAPI { ruby = self: self.on "ruby"; })
+    (i: i.addAPI { python = self: self.on "python"; })
+    (i: i.addAPI { old-school = self: self.off "copilot"; })
+    (i: i.addAPI { vim-btw = self: self.exclusive "vim" "emacs"; })
+  ];
+
+  hasDirMatching = self: re: self.matching ".*/.*?${re}.*/.*";
 
-  root = inputs.import-tree.addPath ./modules;
+  on = self: flagName: self.hasDirMatching "\+${flagName}";
+  off = self: flagName: self.hasDirMatching "\-${flagName}";
 
-  on = flag: tree: tree.filter (lib.hasInfix "/+${flag}/");
-  off = flag: tree: tree.filter (lib.hasInfix "/-${flag}/");
+  exclusive = self: onFlag: offFlag: lib.pipe self [
+    (self: self.on onFlag)
+    (self: self.off offFlag)
+  ];
 in
 {
   inherit flake;
@@ -314,15 +356,16 @@ Users of such distribution can do:
 ```nix
 # consumer flakeModule
 {inputs, lib, ...}: let
-  inherit (inputs.editor-distro.lib.trees) on off root;
+  ed-tree = inputs.editor-distro.lib.modules-tree;
 in {
   imports = [
-    # files inside +vim -emacs directories
-    (lib.pipe root [(on "vim") (off "emacs") (i: i.result)])
+    (ed-tree.vim-btw.old-school.on "rust")
   ];
 }
 ```
 
+</details>
+
 ## Testing
 
 `import-tree` uses [`checkmate`](https://github.com/vic/checkmate) for testing.
diff --git a/checkmate.nix b/checkmate.nix
index 883aa39..03fa790 100644
--- a/checkmate.nix
+++ b/checkmate.nix
@@ -106,6 +106,26 @@ in
           ];
         };
 
+        addAPI."test extends the API available on an import-tree object" = {
+          expr =
+            let
+              extended = lit.addAPI { helloOption = self: self.addPath ./tree/modules/hello-option; };
+            in
+            extended.helloOption.leafs.result;
+          expected = [ ./tree/modules/hello-option/mod.nix ];
+        };
+
+        addAPI."test preserves previous API extensions on an import-tree object" = {
+          expr =
+            let
+              first = lit.addAPI { helloOption = self: self.addPath ./tree/modules/hello-option; };
+              second = first.addAPI { helloWorld = self: self.addPath ./tree/modules/hello-world; };
+              extended = second.addAPI { res = self: self.helloOption.leafs.result; };
+            in
+            extended.res;
+          expected = [ ./tree/modules/hello-option/mod.nix ];
+        };
+
         pipeTo."test pipes list into a function" = {
           expr = (lit.mapWith lib.pathType).pipeTo (lib.length) ./tree/x;
           expected = 1;
diff --git a/default.nix b/default.nix
index 346dd79..cd923e2 100644
--- a/default.nix
+++ b/default.nix
@@ -73,28 +73,36 @@ let
     let
       __config = {
         # Accumulated configuration
+        api = { };
         mapf = (i: i);
         filterf = _: true;
         paths = [ ];
 
-        __functor = self: f: {
-          __config = (f self);
-          __functor = functor;
+        __functor =
+          self: f:
+          let
+            __config = (f self);
+          in
+          __config.api
+          // {
+            inherit __config;
+            __functor = functor;
 
-          # Configuration updates (accumulating)
-          filtered = filterf: self (c: mapAttr (f c) "filterf" (and filterf));
-          matching = regex: self (c: mapAttr (f c) "filterf" (and (matchesRegex regex)));
-          mapWith = mapf: self (c: mapAttr (f c) "mapf" (compose mapf));
-          addPath = path: self (c: mapAttr (f c) "paths" (p: p ++ [ path ]));
+            # Configuration updates (accumulating)
+            filtered = filterf: self (c: mapAttr (f c) "filterf" (and filterf));
+            matching = regex: self (c: mapAttr (f c) "filterf" (and (matchesRegex regex)));
+            mapWith = mapf: self (c: mapAttr (f c) "mapf" (compose mapf));
+            addPath = path: self (c: mapAttr (f c) "paths" (p: p ++ [ path ]));
+            addAPI = api: self (c: mapAttr (f c) "api" (a: a // builtins.mapAttrs (_: g: g (self f)) api));
 
-          # Configuration updates (non-accumulating)
-          withLib = lib: self (c: (f c) // { inherit lib; });
-          pipeTo = pipef: self (c: (f c) // { inherit pipef; });
-          leafs = self (c: (f c) // { pipef = (i: i); });
+            # Configuration updates (non-accumulating)
+            withLib = lib: self (c: (f c) // { inherit lib; });
+            pipeTo = pipef: self (c: (f c) // { inherit pipef; });
+            leafs = self (c: (f c) // { pipef = (i: i); });
 
-          # Applies empty (for already path-configured trees)
-          result = (self f) [ ];
-        };
+            # Applies empty (for already path-configured trees)
+            result = (self f) [ ];
+          };
       };
     in
     __config (c: c);