diff --git a/test/custom.js b/test/custom.js
new file mode 100644
index 0000000..3ac22e9
--- /dev/null
+++ b/test/custom.js
@@ -0,0 +1,21 @@
+import assert from 'assert'
+import math from '../picomath.js'
+import picoInstance from '../picomathInstance.js'
+import createNumber from '../number/all.js'
+import createComplex from '../complex/all.js'
+
+describe('Custom instances', () => {
+   it("doesn't matter what order you assemble", () => {
+      const bw = picoInstance('backwards')
+      createComplex(bw)
+      createNumber(bw)
+      
+      assert.strictEqual(bw.subtract(16, bw.add(3,4,2)), 7)
+      assert.strictEqual(bw.negate(bw.number('8')), -8)
+      assert.deepStrictEqual(bw.complex(2,3), {re: 2, im: 3})
+      assert.deepStrictEqual(
+         bw.subtract(16, bw.add(3, bw.complex(0,4), 2)),
+         math.complex(11, -4)) // note both instances coexist
+      assert.deepStrictEqual(bw.negate(math.complex(3, '8')).im, -8)
+   })
+})