diff --git a/complex/extendByComplex.js b/complex/extendByComplex.js
new file mode 100644
index 0000000..33ce590
--- /dev/null
+++ b/complex/extendByComplex.js
@@ -0,0 +1,23 @@
+import { isTF } from '../poortf.js'
+import createComplex from './complex.js'
+
+/* Add all the complex methods for the number methods that are
+ * already in the instance: */
+export default async function extend(pmath) {
+   for (const item in pmath) {
+      if (isTF(pmath[item])) {
+         // might be a number method, try importing
+         const modulePath = `./${item}.js`
+         try {
+            const { default: create } = await import(modulePath)
+            create(pmath)
+         } catch (err) {
+            // Guess it wasn't a number method; no worries
+         }
+      }
+   }
+   createComplex(pmath)
+}
+
+      
+            
diff --git a/poortf.js b/poortf.js
index cd2f8f5..0b212d7 100644
--- a/poortf.js
+++ b/poortf.js
@@ -25,3 +25,7 @@ export default function poortf (name, imps) {
 
    return fn
 }
+
+export function isTF(x) {
+   return typeof x === 'function' && 'imps' in x && Array.isArray(x.imps)
+}
diff --git a/test/custom.js b/test/custom.js
index 3ac22e9..c14e849 100644
--- a/test/custom.js
+++ b/test/custom.js
@@ -1,14 +1,17 @@
 import assert from 'assert'
 import math from '../picomath.js'
 import picoInstance from '../picomathInstance.js'
-import createNumber from '../number/all.js'
+import createNumbers from '../number/all.js'
+import createNumber from '../number/number.js'
+import createAdd from '../number/add.js'
 import createComplex from '../complex/all.js'
+import extendByComplex from '../complex/extendByComplex.js'
 
 describe('Custom instances', () => {
    it("doesn't matter what order you assemble", () => {
       const bw = picoInstance('backwards')
       createComplex(bw)
-      createNumber(bw)
+      createNumbers(bw)
       
       assert.strictEqual(bw.subtract(16, bw.add(3,4,2)), 7)
       assert.strictEqual(bw.negate(bw.number('8')), -8)
@@ -18,4 +21,19 @@ describe('Custom instances', () => {
          math.complex(11, -4)) // note both instances coexist
       assert.deepStrictEqual(bw.negate(math.complex(3, '8')).im, -8)
    })
+
+   it("can selectively import in cute ways", async function () {
+      const cherry = picoInstance('cherry')
+      createNumber(cherry)
+      createAdd(cherry)
+      await extendByComplex(cherry)
+      // Now we have an instance that supports addition for number and complex
+      // and little else:
+      assert.strictEqual(cherry.add(3, 4, 2), 9)
+      assert.deepStrictEqual(
+         cherry.add(cherry.complex(3, 3), 4, cherry.complex(2, 2)),
+         math.complex(9,5))
+      assert.strictEqual('subtract' in cherry, false)
+      assert.strictEqual('negate' in cherry, false)
+   })
 })