fix: Prevent PocomathInstance from clobbering its own methods

Resolves #6.

src/core/PocomathInstance.mjs

@@ -2,6 +2,12 @@
 import typed from 'typed-function'
 
 export default class PocomathInstance {
+   /* Disallowed names for ops; beware, this is slightly non-DRY
+    * in that if a new top-level PocomathInstance method is added, its name
+    * must be added to this list.
+    */
+   static reserved = new Set(['install'])
+
    constructor(name) {
       this.name = name
       this._imps = {}
@@ -48,6 +54,15 @@ export default class PocomathInstance {
 
    /* Used internally by install, see the documentation there */
    _installOp(name, implementations) {
+      if (name.charAt(0) === '_') {
+         throw new SyntaxError(
+            `Pocomath: Cannot install ${name}, `
+               + 'initial _ reserved for internal use.')
+      }
+      if (PocomathInstance.reserved.has(name)) {
+         throw new SyntaxError(
+            `Pocomath: the meaning of function '${name}' cannot be modified.`)
+      }
       // new implementations, so set the op up to lazily recreate itself
       this._invalidate(name)
       const opImps = this._imps[name]

test/core/_PocomathInstance.mjs

@@ -1,13 +1,21 @@
 import assert from 'assert'
 import PocomathInstance from '../../src/core/PocomathInstance.mjs'
 
+const pi = new PocomathInstance('dummy')
 describe('PocomathInstance', () => {
    it('creates an instance that can define typed-functions', () => {
-      const pi = new PocomathInstance('dummy')
-      pi.install({'add': {'any,any': [[], (a,b) => a+b]}})
+      pi.install({add: {'any,any': [[], (a,b) => a+b]}})
       assert.strictEqual(pi.add(2,2), 4)
       assert.strictEqual(pi.add('Kilroy', 17), 'Kilroy17')
       assert.strictEqual(pi.add(1, undefined), NaN)
       assert.throws(() => pi.add(1), TypeError)
    })
+
+   it('reserves certain function names', () => {
+      assert.throws(
+         () => pi.install({install: {any: [[], x => x]}}), SyntaxError)
+      assert.throws(
+         () => pi.install({_foo: {any: [[], x => x]}}), SyntaxError)
+   })
+
 })