diff --git a/PocomathInstance.mjs b/PocomathInstance.mjs
index 59e6e2f..facdfb4 100644
--- a/PocomathInstance.mjs
+++ b/PocomathInstance.mjs
@@ -5,6 +5,7 @@ export default class PocomathInstance {
    constructor(name) {
       this.name = name
       this._imps = {}
+      this._affects = {}
    }
     
    /**
@@ -46,6 +47,14 @@ export default class PocomathInstance {
                `Conflicting definitions of ${signature} for ${name}`)
          } else {
             opImps[signature] = implementations[signature]
+            for (const dep of implementations[signature][0]) {
+               const depname = dep.split('(', 1)[0]
+               if (depname === 'self') continue
+               if (!(depname in this._affects)) {
+                  this._affects[depname] = new Set()
+               }
+               this._affects[depname].add(name)
+            }
          }
       }
    }
@@ -62,6 +71,11 @@ export default class PocomathInstance {
       if (!(name in this._imps)) {
          this._imps[name] = {}
       }
+      if (name in this._affects) {
+         for (const ancestor of this._affects[name]) {
+            this._invalidate(ancestor)
+         }
+      }
    }
 
    /**
diff --git a/complex/negate.mjs b/complex/negate.mjs
index 1b8a189..429a575 100644
--- a/complex/negate.mjs
+++ b/complex/negate.mjs
@@ -1,4 +1,8 @@
-import './Complex.mjs'
+import {numComplex} from './Complex.mjs'
 export const negate = {
-   Complex: [['self'], ref => z => ({re: ref.self(z.re), im: ref.self(z.im)})]
+   /* need a "base case" to avoid infinite self-reference */
+   Complex: [['self'], ref => z => {
+      if (numComplex(z)) return {re: -z.re, im: -z.im}
+      return {re: ref.self(z.re), im: ref.self(z.im)}
+   }]
 }
diff --git a/test/custom.mjs b/test/custom.mjs
index 807fefc..78dd8db 100644
--- a/test/custom.mjs
+++ b/test/custom.mjs
@@ -4,12 +4,16 @@ import typed from 'typed-function'
 import PocomathInstance from '../PocomathInstance.mjs'
 import * as numbers from '../number/all.mjs'
 import * as complex from '../complex/all.mjs'
+import * as complexAdd from '../complex/add.mjs'
+import * as complexNegate from '../complex/negate.mjs'
 
 const bw = new PocomathInstance('backwards')
 describe('A custom instance', () => {
    it("works when partially assembled", () => {
       bw.install(complex)
       assert.deepStrictEqual(bw.add(2, bw.complex(0, 3)), {re: 2, im: 3})
+      assert.deepStrictEqual(bw.negate(2), bw.complex(-2,-0))
+      assert.deepStrictEqual(bw.subtract(2, bw.complex(0, 3)), {re: 2, im: -3})
    })
 
    it("can be assembled in any order", () => {
@@ -23,4 +27,15 @@ describe('A custom instance', () => {
          math.complex(11, -4)) // note both instances coexist
       assert.deepStrictEqual(bw.negate(math.complex(3, '8')).im, -8)
    })
+
+   it("can be assembled piecemeal", () => {
+      const pm = new PocomathInstance('piecemeal')
+      pm.install(numbers)
+      assert.strictEqual(pm.subtract(5, 10), -5)
+      pm.install(complexAdd)
+      pm.install(complexNegate)
+      // Should be enough to allow complex subtraction, as subtract is generic
+      assert.deepStrictEqual(
+         pm.subtract({re:5, im:0}, {re:10, im:1}), {re:-5, im: -1})
+   })
 })