diff --git a/src/complex/abs.mjs b/src/complex/abs.mjs
new file mode 100644
index 0000000..20ab809
--- /dev/null
+++ b/src/complex/abs.mjs
@@ -0,0 +1,5 @@
+export {Types} from './Types/Complex.mjs'
+
+export const abs = {Complex: ({sqrt, add, multiply}) => z => {
+    return sqrt(add(multiply(z.re, z.re), multiply(z.im, z.im)))
+}}
diff --git a/src/complex/all.mjs b/src/complex/all.mjs
index d5d5bda..2c3946d 100644
--- a/src/complex/all.mjs
+++ b/src/complex/all.mjs
@@ -1,5 +1,2 @@
-export {Types} from './Types/Complex.mjs'
-export {complex} from './complex.mjs'
-export {add} from './add.mjs'
-export {negate} from './negate.mjs'
-export {subtract} from '../generic/subtract.mjs'
+export * from './native.mjs'
+export * from '../generic/arithmetic.mjs'
diff --git a/src/complex/native.mjs b/src/complex/native.mjs
new file mode 100644
index 0000000..aec0868
--- /dev/null
+++ b/src/complex/native.mjs
@@ -0,0 +1,8 @@
+export {Types} from './Types/Complex.mjs'
+
+export {abs} from './abs.mjs'
+export {add} from './add.mjs'
+export {complex} from './complex.mjs'
+export {negate} from './negate.mjs'
+export {sqrt} from './sqrt.mjs'
+
diff --git a/src/complex/sqrt.mjs b/src/complex/sqrt.mjs
new file mode 100644
index 0000000..93cd10a
--- /dev/null
+++ b/src/complex/sqrt.mjs
@@ -0,0 +1,37 @@
+export { Types } from './Types/Complex.mjs'
+
+export const sqrt = {
+   Complex: ({
+      config,
+      complex,
+      multiply,
+      sign,
+      self,
+      divide,
+      add,
+      'abs(Complex)': abs,
+      subtract
+   }) => {
+      if (config.predictable) {
+         return z => {
+            const imSign = sign(z.im)
+            const reSign = sign(z.re)
+            if (imSign === 0 && reSign === 1) return complex(self(z.re))
+            return complex(
+               multiply(sign(z.im), self(divide(add(abs(z),z.re), 2))),
+               self(divide(subtract(abs(z),z.re), 2))
+            )
+         }
+      }
+      return z => {
+         const imSign = sign(z.im)
+         const reSign = sign(z.re)
+         if (imSign === 0 && reSign === 1) return self(z.re)
+         return complex(
+            multiply(sign(z.im), self(divide(add(abs(z),z.re), 2))),
+            self(divide(subtract(abs(z),z.re), 2))
+         )
+      }
+   }
+}
+
diff --git a/src/core/PocomathInstance.mjs b/src/core/PocomathInstance.mjs
index 8e66280..646bd62 100644
--- a/src/core/PocomathInstance.mjs
+++ b/src/core/PocomathInstance.mjs
@@ -8,7 +8,7 @@ export default class PocomathInstance {
     * in that if a new top-level PocomathInstance method is added, its name
     * must be added to this list.
     */
-   static reserved = new Set(['install', 'importDependencies'])
+   static reserved = new Set(['config', 'importDependencies', 'install', 'name'])
 
    constructor(name) {
       this.name = name
@@ -17,6 +17,19 @@ export default class PocomathInstance {
       this._typed = typed.create()
       this._typed.clear()
       this.Types = {any: {}} // dummy entry to track the default 'any' type
+      this._doomed = new Set() // for detecting circular reference
+      this._config = {predictable: false}
+      const self = this
+      this.config = new Proxy(this._config, {
+         get: (target, property) => target[property],
+         set: (target, property, value) => {
+            if (value !== target[property]) {
+               target[property] = value
+               self._invalidateDependents('config')
+            }
+            return true // successful
+         }
+      })
    }
 
    /**
@@ -80,6 +93,8 @@ export default class PocomathInstance {
     * @param {string[]} types  A list of type names
     */
    async importDependencies(types) {
+      const typeSet = new Set(types)
+      typeSet.add('generic')
       const doneSet = new Set(['self']) // nothing to do for self dependencies
       while (true) {
          const requiredSet = new Set()
@@ -96,7 +111,7 @@ export default class PocomathInstance {
          }
          if (requiredSet.size === 0) break
          for (const name of requiredSet) {
-            for (const type of types) {
+            for (const type of typeSet) {
                try {
                   const modName = `../${type}/${name}.mjs`
                   const mod = await import(modName)
@@ -145,7 +160,8 @@ export default class PocomathInstance {
             }
          }
          this.Types[type] = spec
-         this._invalidate(':' + type) // rebundle anything that uses the new type
+         // rebundle anything that uses the new type:
+         this._invalidateDependents(':' + type)
       }
    }
          
@@ -198,14 +214,27 @@ export default class PocomathInstance {
     * and if it has no implementations so far, set them up.
     */
    _invalidate(name) {
+      if (this._doomed.has(name)) {
+         /* In the midst of a circular invalidation, so do nothing */
+         return
+      }
+      if (!(name in this._imps)) {
+         this._imps[name] = {}
+      }
+      this._doomed.add(name)
+      this._invalidateDependents(name)
+      this._doomed.delete(name)
       const self = this
       Object.defineProperty(this, name, {
          configurable: true,
          get: () => self._bundle(name)
       })
-      if (!(name in this._imps)) {
-         this._imps[name] = {}
-      }
+   }
+
+   /**
+    * Invalidate all the dependents of a given property of the instance
+    */
+   _invalidateDependents(name) {
       if (name in this._affects) {
          for (const ancestor of this._affects[name]) {
             this._invalidate(ancestor)
@@ -229,29 +258,69 @@ export default class PocomathInstance {
             `Every implementation for ${name} uses an undefined type;\n`
                + `    signatures: ${Object.keys(imps)}`)
       }
+      Object.defineProperty(this, name, {configurable: true, value: 'limbo'})
       const tf_imps = {}
       for (const [signature, {uses, does}] of usableEntries) {
          if (uses.length === 0) {
             tf_imps[signature] = does()
          } else {
             const refs = {}
-            let self_referential = false
+            let full_self_referential = false
+            let part_self_references = []
             for (const dep of uses) {
-               // TODO: handle signature-specific dependencies
-               if (dep.includes('(')) {
-                  throw new Error('signature specific reference unimplemented')
-               }
-               if (dep === 'self') {
-                  self_referential = true
+               const [func, needsig] = dep.split(/[()]/)
+               if (func === 'self') {
+                  if (needsig) {
+                     if (full_self_referential) {
+                        throw new SyntaxError(
+                           'typed-function does not support mixed full and '
+                              + 'partial self-reference')
+                     }
+                     if (subsetOfKeys(typesOfSignature(needsig), this.Types)) {
+                        part_self_references.push(needsig)
+                     }
+                  } else {
+                     if (part_self_references.length) {
+                        throw new SyntaxError(
+                           'typed-function does not support mixed full and '
+                              + 'partial self-reference')
+                     }
+                     full_self_referential = true
+                  }
                } else {
-                  refs[dep] = this[dep] // assume acyclic for now
+                  if (this[func] === 'limbo') {
+                     /* We are in the midst of bundling func, so have to use
+                      * an indirect reference to func. And given that, there's
+                      * really no helpful way to extract a specific signature
+                      */
+                     const self = this
+                     refs[dep] = function () { // is this the most efficient?
+                        return self[func].apply(this, arguments)
+                     }
+                  } else {
+                     // can bundle up func, and grab its signature if need be
+                     let destination = this[func]
+                     if (needsig) {
+                        destination = this._typed.find(destination, needsig)
+                     }
+                     refs[dep] = destination
+                  }
                }
             }
-            if (self_referential) {
+            if (full_self_referential) {
                tf_imps[signature] = this._typed.referToSelf(self => {
                   refs.self = self
                   return does(refs)
                })
+            } else if (part_self_references.length) {
+               tf_imps[signature] = this._typed.referTo(
+                  ...part_self_references, (...impls) => {
+                     for (let i = 0; i < part_self_references.length; ++i) {
+                        refs[`self(${part_self_references[i]})`] = impls[i]
+                     }
+                     return does(refs)
+                  }
+               )
             } else {
                tf_imps[signature] = does(refs)
             }
diff --git a/src/core/dependencyExtractor.mjs b/src/core/dependencyExtractor.mjs
index fbf5611..1b1091c 100644
--- a/src/core/dependencyExtractor.mjs
+++ b/src/core/dependencyExtractor.mjs
@@ -4,6 +4,9 @@
  */
 export default function dependencyExtractor(destinationSet) {
    return new Proxy({}, {
-      get: (target, property) => { destinationSet.add(property) }
+      get: (target, property) => {
+         destinationSet.add(property)
+         return {}
+      }
    })
 }
diff --git a/src/generic/all.mjs b/src/generic/all.mjs
new file mode 100644
index 0000000..db678dd
--- /dev/null
+++ b/src/generic/all.mjs
@@ -0,0 +1 @@
+export * from './arithmetic.mjs'
diff --git a/src/generic/arithmetic.mjs b/src/generic/arithmetic.mjs
new file mode 100644
index 0000000..2787c41
--- /dev/null
+++ b/src/generic/arithmetic.mjs
@@ -0,0 +1,3 @@
+export {divide} from './divide.mjs'
+export {sign} from './sign.mjs'
+export {subtract} from './subtract.mjs'
diff --git a/src/generic/divide.mjs b/src/generic/divide.mjs
new file mode 100644
index 0000000..4dcfc89
--- /dev/null
+++ b/src/generic/divide.mjs
@@ -0,0 +1,4 @@
+export const divide = {
+   'any,any': ({multiply, invert}) => (x, y) => multiply(x, invert(y))
+}
+
diff --git a/src/generic/sign.mjs b/src/generic/sign.mjs
new file mode 100644
index 0000000..c8133fd
--- /dev/null
+++ b/src/generic/sign.mjs
@@ -0,0 +1,6 @@
+export const sign = {
+   any: ({negate, divide, abs}) => x => {
+      if (x === negate(x)) return x // zero
+      return divide(x, abs(x))
+   }
+}
diff --git a/src/number/abs.mjs b/src/number/abs.mjs
new file mode 100644
index 0000000..1613356
--- /dev/null
+++ b/src/number/abs.mjs
@@ -0,0 +1,3 @@
+export {Types} from './Types/number.mjs'
+
+export const abs = {number: () => n => Math.abs(n)}
diff --git a/src/number/all.mjs b/src/number/all.mjs
index bf737f5..e437adb 100644
--- a/src/number/all.mjs
+++ b/src/number/all.mjs
@@ -1,4 +1,4 @@
 export {Types} from './Types/number.mjs'
-export {add} from './add.mjs'
-export {negate} from './negate.mjs'
-export {subtract} from '../generic/subtract.mjs'
+
+export * from './native.mjs'
+export * from '../generic/arithmetic.mjs'
diff --git a/src/number/invert.mjs b/src/number/invert.mjs
new file mode 100644
index 0000000..11d24c3
--- /dev/null
+++ b/src/number/invert.mjs
@@ -0,0 +1,3 @@
+export {Types} from './Types/number.mjs'
+
+export const invert = {number: () => n => 1/n}
diff --git a/src/number/multiply.mjs b/src/number/multiply.mjs
new file mode 100644
index 0000000..dff6611
--- /dev/null
+++ b/src/number/multiply.mjs
@@ -0,0 +1,5 @@
+export {Types} from './Types/number.mjs'
+
+export const multiply = {
+   '...number': () => multiplicands => multiplicands.reduce((x,y) => x*y, 1),
+}
diff --git a/src/number/native.js b/src/number/native.js
new file mode 100644
index 0000000..208d8a2
--- /dev/null
+++ b/src/number/native.js
@@ -0,0 +1,4 @@
+export {Types} from './Types/number.mjs'
+export {add} from './add.mjs'
+export {negate} from './negate.mjs'
+export {sqrt} from './sqrt.mjs'
diff --git a/src/number/native.mjs b/src/number/native.mjs
new file mode 100644
index 0000000..99807de
--- /dev/null
+++ b/src/number/native.mjs
@@ -0,0 +1,8 @@
+export {Types} from './Types/number.mjs'
+
+export {abs} from './abs.mjs'
+export {add} from './add.mjs'
+export {invert} from './invert.mjs'
+export {multiply} from './multiply.mjs'
+export {negate} from './negate.mjs'
+export {sqrt} from './sqrt.mjs'
diff --git a/src/number/sqrt.mjs b/src/number/sqrt.mjs
new file mode 100644
index 0000000..179aa0a
--- /dev/null
+++ b/src/number/sqrt.mjs
@@ -0,0 +1,14 @@
+export { Types } from './Types/number.mjs'
+
+export const sqrt = {
+   number: ({config, complex, 'self(Complex)': complexSqrt}) => {
+      if (config.predictable || !complexSqrt) {
+         return n => isNaN(n) ? NaN : Math.sqrt(n)
+      }
+      return n => {
+         if (isNaN(n)) return NaN
+         if (n >= 0) return Math.sqrt(n)
+         return complexSqrt(complex(n))
+      }
+   }
+}
diff --git a/src/pocomath.mjs b/src/pocomath.mjs
index c5a81d4..e0015a9 100644
--- a/src/pocomath.mjs
+++ b/src/pocomath.mjs
@@ -1,12 +1,14 @@
 /* Core of pocomath: generates the default instance */
 import PocomathInstance from './core/PocomathInstance.mjs'
-import * as numbers from './number/all.mjs'
+import * as numbers from './number/native.mjs'
 import * as bigints from './bigint/all.mjs'
-import * as complex from './complex/all.mjs'
+import * as complex from './complex/native.mjs'
+import * as generic from './generic/all.mjs'
 
 const math = new PocomathInstance('math')
 math.install(numbers)
 math.install(bigints)
 math.install(complex)
+math.install(generic)
 
 export default math
diff --git a/test/complex/## b/test/complex/##
new file mode 100755
index 0000000..e69de29
diff --git a/test/complex/_all.mjs b/test/complex/_all.mjs
new file mode 100644
index 0000000..9d09222
--- /dev/null
+++ b/test/complex/_all.mjs
@@ -0,0 +1,29 @@
+import assert from 'assert'
+import math from '../../src/pocomath.mjs'
+import PocomathInstance from '../../src/core/PocomathInstance.mjs'
+import * as complexSqrt from '../../src/complex/sqrt.mjs'
+
+describe('complex', () => {
+   it('supports sqrt', () => {
+      assert.deepStrictEqual(math.sqrt(math.complex(1,0)), 1)
+      assert.deepStrictEqual(
+         math.sqrt(math.complex(0,1)),
+         math.complex(math.sqrt(0.5), math.sqrt(0.5)))
+      math.config.predictable = true
+      assert.deepStrictEqual(math.sqrt(math.complex(1,0)), math.complex(1,0))
+      assert.deepStrictEqual(
+         math.sqrt(math.complex(0,1)),
+         math.complex(math.sqrt(0.5), math.sqrt(0.5)))
+      math.config.predictable = false
+   })
+
+   it('can bundle sqrt', async function () {
+      const ms = new PocomathInstance('Minimal Sqrt')
+      ms.install(complexSqrt)
+      await ms.importDependencies(['number', 'complex'])
+      assert.deepStrictEqual(
+         ms.sqrt(math.complex(0, -1)),
+         math.complex(ms.negate(ms.sqrt(0.5)), ms.sqrt(0.5)))
+   })
+
+})
diff --git a/test/number/_all.mjs b/test/number/_all.mjs
new file mode 100644
index 0000000..b334fef
--- /dev/null
+++ b/test/number/_all.mjs
@@ -0,0 +1,29 @@
+import assert from 'assert'
+import math from '../../src/pocomath.mjs'
+import PocomathInstance from '../../src/core/PocomathInstance.mjs'
+import * as numberSqrt from '../../src/number/sqrt.mjs'
+import * as complex from '../../src/complex/all.mjs'
+import * as numbers from '../../src/number/all.mjs'
+describe('number', () => {
+   it('supports sqrt', () => {
+      assert.strictEqual(math.sqrt(4), 2)
+      assert.strictEqual(math.sqrt(NaN), NaN)
+      assert.strictEqual(math.sqrt(2.25), 1.5)
+      assert.deepStrictEqual(math.sqrt(-9), math.complex(0, 3))
+      math.config.predictable = true
+      assert.strictEqual(math.sqrt(-9), NaN)
+      math.config.predictable = false
+      assert.deepStrictEqual(math.sqrt(-0.25), math.complex(0, 0.5))
+   })
+
+   it('supports sqrt by itself', () => {
+      const no = new PocomathInstance('Numbers Only')
+      no.install(numberSqrt)
+      assert.strictEqual(no.sqrt(2.56), 1.6)
+      assert.strictEqual(no.sqrt(-17), NaN)
+      no.install(complex)
+      no.install(numbers)
+      assert.deepStrictEqual(no.sqrt(-16), no.complex(0,4))
+   })
+
+})