diff --git a/complex/add.js b/complex/add.js
index 1293a14..a831574 100644
--- a/complex/add.js
+++ b/complex/add.js
@@ -2,6 +2,7 @@ import { anyComplex } from './complex.js'
 
 export default function create(pmath) {
     const complex = pmath('complex')
+    const resolve = pmath.config.resolveComplex
     return pmath('add', [anyComplex, // naive, but this is just a P-o-C
        (...addends) => {
            let sum = complex(addends[0])
@@ -10,7 +11,9 @@ export default function create(pmath) {
                sum.re += addend.re
                sum.im += addend.im
            }
+           if (resolve && Math.abs(sum.im/sum.re) < 1e-10) return sum.re
            return sum
-       }])
+       }],
+    create, pmath) // register ourselves for invalidation and reconfiguration
 }
                          
diff --git a/generic/subtract.js b/generic/subtract.js
index 236885c..87745e0 100644
--- a/generic/subtract.js
+++ b/generic/subtract.js
@@ -1,9 +1,9 @@
 export default function create(pmath) {
    const add = pmath('add')
    const negate = pmath('negate')
-   if (!pmath.subtract) { // avoid double definition at cost of extensibility
-      pmath('subtract', [args => args.length === 2,
-         (x, y) => add(x, negate(y))])
-   }
+   return pmath(
+      'subtract',
+      [args => args.length === 2, (x, y) => add(x, negate(y))],
+      create, pmath)
    return pmath.subtract
 }
diff --git a/picomathInstance.js b/picomathInstance.js
index 04afd13..90e4041 100644
--- a/picomathInstance.js
+++ b/picomathInstance.js
@@ -1,23 +1,44 @@
 /* Core of picomath: generates an instance */
 import poortf from './poortf.js'
 
-export default function picomathInstance (instName) {
-    /* Since we have to do it all the time, when we call a picomath instance
-     * as a function, it takes a name and 0 or more implementations add adds
-     * them to its poortf property named name, returning that property value.
-     */
-    function fn (name, imps) {
-        if (name in fn) {
-            fn[name].addImps(imps)
-        } else {
-            fn[name] = poortf(name, imps)
-        }
-        return fn[name]
-    }
+export default function picomathInstance (instName, config) {
+   /* Since we have to do it all the time, when we call a picomath instance
+    * as a function, it takes a name and 0 or more implementations add adds
+    * them to its poortf property named name, returning that property value.
+    */
+   function fn (name, imps, author, data) {
+      if (name in fn) {
+         fn[name].addImps(imps, author, data)
+      } else {
+         fn[name] = poortf(name, imps, author, data)
+      }
+      /* For PoC, just assume only reason to provide author info
+       * is to be invalidated when config changes
+       */
+      if (author) {
+         if (!fn.configUsers.has(author)) {
+            fn.configUsers.set(author, new Set())
+         }
+         fn.configUsers.get(author).add(name)
+      }
+      return fn[name]
+   }
 
-    Object.defineProperty(fn, 'name', {value: instName})
-
-    return fn
+   Object.defineProperty(fn, 'name', {value: instName})
+   // There is an issue below of possible collision between the following
+   // property names and operation names. Since it would not be too hard to
+   // solve, we won't worry about it in this PoC.
+   fn.config = config || { resolveComplex: false } // primitive default for POC
+   fn.configUsers = new Map()
+   fn.reconfigure = config => {
+      for (const [author, names] of fn.configUsers.entries()) {
+         for (const name of names) {
+            fn[name].invalidate(author)
+         }
+      }
+      fn.config = config
+   }
+   return fn
 }
 
     
diff --git a/poortf.js b/poortf.js
index 0b212d7..68936ab 100644
--- a/poortf.js
+++ b/poortf.js
@@ -1,13 +1,33 @@
 /* Totally minimal "typed functions" */
 
-const addImps = (dest, imps) => {
+/* helper: dest is really a TF and imps is either nothing, an imp, or
+ * an array of implementations. author is the entity responsible for these
+ * implementations.
+ */
+const addImps = (dest, imps, author) => {
    if (imps) {
       if (!Array.isArray(imps[0])) imps = [imps]
-      for (const imp of imps) dest.push(imp)
+   } else {
+      imps = []
    }
+   if (author) {
+      if (dest.authors.has(author)) {
+         const [count, index] = dest.authors.get(author)
+         if (count) dest.imps.splice(index, count)
+      }
+      dest.authors.set(author, [imps.length, dest.imps.length])
+   }
+   for (const imp of imps) dest.imps.push(imp)
 }
 
-export default function poortf (name, imps) {
+/* Create a TF, optionally adding some initial imps and further optionally
+ * registering them to an author. There are two advantages to author
+ * registration: (1) if the same author adds implementations, the prior imps
+ * will be deleted, and (2) the author can be invalidated, and the TF will
+ * lazily call the author back with the given data the next time it is 
+ * called, so that it can re-add the imps.
+ */
+export default function poortf (name, imps, author, data) {
    /* This is the (function) object we will return */
    function fn () {
       for (const imp of fn.imps) {
@@ -20,9 +40,40 @@ export default function poortf (name, imps) {
    /* Now dress it up for use */
    Object.defineProperty(fn, 'name', {value: name})
    fn.imps = []
-   addImps(fn.imps, imps)
-   fn.addImps = newI => addImps(fn.imps, newI)
-
+   fn.authors = new Map() // tracks who made each implementation
+   fn.authorData = new Map() // tracks author callback data
+   addImps(fn, imps, author)
+   if (author) fn.authorData.set(author, data)
+   fn.addImps = (newI, author, data) => {
+      addImps(fn, newI, author)
+      if (author) fn.authorData.set(author, data)
+   }
+   fn.invalidate = author => {
+      addImps(fn, null, author) // deletes the author's imps
+      fn.unshiftImp([() => true, (...args) => fn.reloadAndRecall(args)])
+   }
+   fn.unshiftImp = newImp => {
+      fn.imps.unshift(newImp)
+      for (const record in fn.authors.values()) {
+         record[1] += 1
+      }
+   }
+   fn.shiftImp = () => {
+      const oldImp = fn.imps.shift()
+      for (const record in fn.authors.values()) {
+         record[1] -= 1
+      }
+   }
+   fn.reloadAndRecall = (argArray) => {
+      fn.shiftImp() // throw ourself away, ha ha
+      for (const author of fn.authorData.keys()) {
+         if (fn.authors.get(author)[0] === 0) {
+            // imps count of 0 means the author was invalidated
+            author(fn.authorData.get(author))
+         }
+      }
+      return fn.apply(null, argArray)
+   }
    return fn
 }
 
diff --git a/test/_picomath.js b/test/_picomath.js
index 84ff19b..d0126e7 100644
--- a/test/_picomath.js
+++ b/test/_picomath.js
@@ -20,4 +20,8 @@ describe('The default full picomath instance "math"', () => {
          math.complex(11, -4))
       assert.deepStrictEqual(math.negate(math.complex(3, '8')).im, -8)
    })
+
+   it('does not double-define subtract', () => {
+      assert.deepStrictEqual(math.subtract.imps.length, 1)
+   })
 })
diff --git a/test/_poortf.js b/test/_poortf.js
index 3984407..a5f9f6f 100644
--- a/test/_poortf.js
+++ b/test/_poortf.js
@@ -21,13 +21,22 @@ describe('poortf', () => {
       assert.throws(() => add('kilroy'), TypeError)
    })
 
+   const defNumber = what => {
+      slate.addImps([args => typeof args[0] === 'number',
+                     n => 'I am not number ' + (what+n)],
+                    defNumber, what+1)
+   }
+   
    it('extends an empty tf', () => {
+      defNumber(2)
+      assert.strictEqual(slate(0, 'was here'), 'I am not number 2')
+      assert.throws(() => slate('Kilroy', 'was here'), TypeError)
       slate.addImps([
          [args => typeof args[0] === 'string', s => s + ' wuz here'],
-         [args => typeof args[0] === 'number', () => 'I am not a number']
+         [args => typeof args[0] === 'boolean', () => 'Maybe I am a number']
       ])
       assert.strictEqual(slate('Kilroy', 'was here'), 'Kilroy wuz here')
-      assert.strictEqual(slate(2, 'was here'), 'I am not a number')
+      assert.strictEqual(slate(true), 'Maybe I am a number')
       assert.throws(() => slate(['Ha!']), TypeError)
    })
 
@@ -36,6 +45,15 @@ describe('poortf', () => {
       assert.strictEqual(add('Kilroy', 23), 'Kilroy23')
       assert.throws(() => add(['Ha!'], 'gotcha'), TypeError)
    })
+   
+   it('can invalidate and lazily reload implementations', () => {
+      slate.invalidate(defNumber)
+      assert.strictEqual(slate.imps.length, 3) // reloader plus two non-number
+      assert.strictEqual(slate(1), 'I am not number 4') // 'what' is now 3!!
+      assert.strictEqual(slate('Yorlik', 7), 'Yorlik wuz here')
+      assert.strictEqual(slate(false), 'Maybe I am a number')
+      assert.strictEqual(slate.imps.length, 3) // reloader gone
+   })
 })
       
       
diff --git a/test/custom.js b/test/custom.js
index c14e849..1107800 100644
--- a/test/custom.js
+++ b/test/custom.js
@@ -36,4 +36,20 @@ describe('Custom instances', () => {
       assert.strictEqual('subtract' in cherry, false)
       assert.strictEqual('negate' in cherry, false)
    })
+
+   const res = picoInstance('resolving', { resolveComplex: true })
+   createNumbers(res)
+   createComplex(res)
+
+   it("can be configured", () => {
+      assert.strictEqual(res.add(res.complex(2,3), res.complex(2,-3)), 4)
+      assert.deepStrictEqual(math.add(math.complex(2,3), math.complex(2,-3)),
+         math.complex(4,0))
+   })
+
+   it("can be reconfigured", () => {
+      res.reconfigure({resolveComplex: false})
+      assert.deepStrictEqual(math.add(math.complex(2,3), math.complex(2,-3)),
+         math.complex(4,0))
+   })
 })