feat: Selective loader that extends all/only existing number methods to complex

Plus a test example where an instance is initialized to have only addition,
  and then that instance is extended to complex numbers, at which point it
  has number and complex addition but not negation or subtraction (at all).

complex/extendByComplex.js

@@ -0,0 +1,23 @@
+import { isTF } from '../poortf.js'
+import createComplex from './complex.js'
+
+/* Add all the complex methods for the number methods that are
+ * already in the instance: */
+export default async function extend(pmath) {
+   for (const item in pmath) {
+      if (isTF(pmath[item])) {
+         // might be a number method, try importing
+         const modulePath = `./${item}.js`
+         try {
+            const { default: create } = await import(modulePath)
+            create(pmath)
+         } catch (err) {
+            // Guess it wasn't a number method; no worries
+         }
+      }
+   }
+   createComplex(pmath)
+}
+
+      
+            

poortf.js

@@ -25,3 +25,7 @@ export default function poortf (name, imps) {
 
    return fn
 }
+
+export function isTF(x) {
+   return typeof x === 'function' && 'imps' in x && Array.isArray(x.imps)
+}

test/custom.js

@@ -1,14 +1,17 @@
 import assert from 'assert'
 import math from '../picomath.js'
 import picoInstance from '../picomathInstance.js'
-import createNumber from '../number/all.js'
+import createNumbers from '../number/all.js'
+import createNumber from '../number/number.js'
+import createAdd from '../number/add.js'
 import createComplex from '../complex/all.js'
+import extendByComplex from '../complex/extendByComplex.js'
 
 describe('Custom instances', () => {
    it("doesn't matter what order you assemble", () => {
       const bw = picoInstance('backwards')
       createComplex(bw)
-      createNumber(bw)
+      createNumbers(bw)
       
       assert.strictEqual(bw.subtract(16, bw.add(3,4,2)), 7)
       assert.strictEqual(bw.negate(bw.number('8')), -8)
@@ -18,4 +21,19 @@ describe('Custom instances', () => {
          math.complex(11, -4)) // note both instances coexist
       assert.deepStrictEqual(bw.negate(math.complex(3, '8')).im, -8)
    })
+
+   it("can selectively import in cute ways", async function () {
+      const cherry = picoInstance('cherry')
+      createNumber(cherry)
+      createAdd(cherry)
+      await extendByComplex(cherry)
+      // Now we have an instance that supports addition for number and complex
+      // and little else:
+      assert.strictEqual(cherry.add(3, 4, 2), 9)
+      assert.deepStrictEqual(
+         cherry.add(cherry.complex(3, 3), 4, cherry.complex(2, 2)),
+         math.complex(9,5))
+      assert.strictEqual('subtract' in cherry, false)
+      assert.strictEqual('negate' in cherry, false)
+   })
 })