diff --git a/src/bigint/all.mjs b/src/bigint/all.mjs
index 74dce18..f9da9f0 100644
--- a/src/bigint/all.mjs
+++ b/src/bigint/all.mjs
@@ -1,5 +1,6 @@
 import PocomathInstance from '../core/PocomathInstance.mjs'
 import * as bigints from './native.mjs'
 import * as generic from '../generic/all.mjs'
+import * as floor from '../ops/floor.mjs'
 
-export default PocomathInstance.merge('bigint', bigints, generic)
+export default PocomathInstance.merge('bigint', bigints, generic, floor)
diff --git a/src/complex/all.mjs b/src/complex/all.mjs
index 5d9f3b3..98eae90 100644
--- a/src/complex/all.mjs
+++ b/src/complex/all.mjs
@@ -1,5 +1,6 @@
 import PocomathInstance from '../core/PocomathInstance.mjs'
 import * as complexes from './native.mjs'
 import * as generic from '../generic/arithmetic.mjs'
+import * as floor from '../ops/floor.mjs'
 
-export default PocomathInstance.merge('complex', complexes, generic)
+export default PocomathInstance.merge('complex', complexes, generic, floor)
diff --git a/src/core/PocomathInstance.mjs b/src/core/PocomathInstance.mjs
index 180ca17..4858a54 100644
--- a/src/core/PocomathInstance.mjs
+++ b/src/core/PocomathInstance.mjs
@@ -471,6 +471,7 @@ export default class PocomathInstance {
          }
 
          for (const instType of instantiationSet) {
+            if (!(instType in this.Types)) continue
             if (this.Types[instType] === anySpec) continue
             const signature =
                   substituteInSig(trimSignature, theTemplateParam, instType)
diff --git a/src/number/all.mjs b/src/number/all.mjs
index 4a1228e..bedaa24 100644
--- a/src/number/all.mjs
+++ b/src/number/all.mjs
@@ -1,6 +1,7 @@
 import PocomathInstance from '../core/PocomathInstance.mjs'
 import * as numbers from './native.mjs'
 import * as generic from '../generic/all.mjs'
+import * as floor from '../ops/floor.mjs'
 
-export default PocomathInstance.merge('number', numbers, generic)
+export default PocomathInstance.merge('number', numbers, generic, floor)
 
diff --git a/src/ops/floor.mjs b/src/ops/floor.mjs
new file mode 100644
index 0000000..98d524a
--- /dev/null
+++ b/src/ops/floor.mjs
@@ -0,0 +1,25 @@
+import {Complex} from '../complex/Types/Complex.mjs'
+
+/* Note we don't **export** any types here, so that only the options
+ * below that correspond to types that have been installed are activated.
+ */
+
+export const floor = {
+   bigint: () => x => x,
+   NumInt: () => x => x, // Because Pocomath isn't part of typed-function, or
+   GaussianInteger: () => x => x, // at least have access to the real
+   // typed-function parse, we unfortunately can't coalesce these into one
+   // entry with type `bigint|NumInt|GaussianInteger` because they couldn't
+   // be separately activated then
+
+   number: ({'equal(number,number)': eq}) => n => {
+      if (eq(n, Math.round(n))) return Math.round(n)
+      return Math.floor(n)
+   },
+
+   Complex: Complex.promoteUnary.Complex,
+
+   // OK to include a type totally not in Pocomath yet, it'll never be
+   // activated.
+   Fraction: ({quotient}) => f => quotient(f.n, f.d),
+}
diff --git a/src/pocomath.mjs b/src/pocomath.mjs
index 65f8103..cb9bf94 100644
--- a/src/pocomath.mjs
+++ b/src/pocomath.mjs
@@ -4,7 +4,9 @@ import * as numbers from './number/native.mjs'
 import * as bigints from './bigint/native.mjs'
 import * as complex from './complex/native.mjs'
 import * as generic from './generic/all.mjs'
+import * as floor from './ops/floor.mjs'
 
-const math = PocomathInstance.merge('math', numbers, bigints, complex, generic)
+const math = PocomathInstance.merge(
+    'math', numbers, bigints, complex, generic, floor)
 
 export default math
diff --git a/test/_pocomath.mjs b/test/_pocomath.mjs
index 1a6fabe..3e6f2fe 100644
--- a/test/_pocomath.mjs
+++ b/test/_pocomath.mjs
@@ -3,12 +3,12 @@ import PocomathInstance from '../src/core/PocomathInstance.mjs'
 import math from '../src/pocomath.mjs'
 
 describe('The default full pocomath instance "math"', () => {
-   it('has no undefined types', () => {
+   it('has no unexpected undefined types', () => {
       const undef = math.undefinedTypes()
       if (undef.length) {
-         console.log('Probable typo: found undefined types', undef)
+         console.log('NOTE: Found undefined types', undef)
       }
-      assert.strictEqual(undef.length, 0)
+      assert.strictEqual(undef.length, 1) // Mentioning 'Fraction' but not using
    })
 
    it('has a built-in typeOf operator', () => {
diff --git a/test/bigint/_all.mjs b/test/bigint/_all.mjs
index 29b3d27..371b2a0 100644
--- a/test/bigint/_all.mjs
+++ b/test/bigint/_all.mjs
@@ -64,4 +64,8 @@ describe('bigint', () => {
       assert.strictEqual(math.lcm(0n, 0n), 0n)
    })
 
+   it('allows floor as a no-op', () => {
+      assert.strictEqual(math.floor(-333n), -333n)
+   })
+
 })
diff --git a/test/complex/_all.mjs b/test/complex/_all.mjs
index ae146b6..e486fc3 100644
--- a/test/complex/_all.mjs
+++ b/test/complex/_all.mjs
@@ -51,4 +51,12 @@ describe('complex', () => {
          math.complex(4n, 5n))
    })
 
+   it('computes floor', () => {
+      assert.deepStrictEqual(
+         math.floor(math.complex(19, 22.7)),
+         math.complex(19, 22))
+      const gi = math.complex(-1n, 1n)
+      assert.strictEqual(math.floor(gi), gi) // literally a no-op
+   })
+
 })
diff --git a/test/custom.mjs b/test/custom.mjs
index 7aef959..ba4c713 100644
--- a/test/custom.mjs
+++ b/test/custom.mjs
@@ -38,12 +38,20 @@ describe('A custom instance', () => {
       const pm = new PocomathInstance('piecemeal')
       pm.install(numbers)
       assert.strictEqual(pm.subtract(5, 10), -5)
+      assert.strictEqual(pm.floor(3.7), 3)
+      assert.throws(() => pm.floor(10n), TypeError)
       pm.install(complexAdd)
       pm.install(complexNegate)
       pm.install(complexComplex)
       // 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})
+         pm.subtract(pm.complex(5, 0), pm.complex(10, 1)),
+         math.complex(-5, -1))
+      // And now floor has been activated for Complex as well, since the type
+      // is present
+      assert.deepStrictEqual(
+         pm.floor(math.complex(1.9, 0)),
+         math.complex(1))
    })
 
    it("can defer definition of (even used) types", () => {
diff --git a/test/number/_all.mjs b/test/number/_all.mjs
index f8e7312..49c7da6 100644
--- a/test/number/_all.mjs
+++ b/test/number/_all.mjs
@@ -37,4 +37,10 @@ describe('number', () => {
       assert.ok(math.largerEq(12.5, math.divide(25,2)))
    })
 
+   it('Computes floor', () => {
+      assert.strictEqual(math.floor(7), 7)
+      assert.strictEqual(math.floor(6.99), 6)
+      assert.strictEqual(math.floor(1-1e-13), 1)
+   })
+
 })