refactor: Add stubs for function templates

The stubs are so far all no-ops so basically at the moment the only
  allowed template parameter 'T' is just a synonym for 'any'. But at least
  this allows us to put the definition of the generic `subtract` into
  the exact form we actually want to support.

src/core/PocomathInstance.mjs

@@ -26,7 +26,10 @@ export default class PocomathInstance {
       this._affects = {}
       this._typed = typed.create()
       this._typed.clear()
-      this.Types = {any: anySpec} // dummy entry to track the default 'any' type
+      /* List of types installed in the instance. We start with just dummies
+       * for the 'any' type and for type parameters:
+       */
+      this.Types = {any: anySpec, T: anySpec}
       this._subtypes = {} // For each type, gives all of its (in)direct subtypes
       this._usedTypes = new Set() // all types that have occurred in a signature
       this._doomed = new Set() // for detecting circular reference
@@ -130,6 +133,7 @@ export default class PocomathInstance {
 
    _installInstance(other) {
       for (const [type, spec] of Object.entries(other.Types)) {
+         if (type === 'any' || this._templateParam(type)) continue
          this.installType(type, spec)
       }
       this._installFunctions(other._imps)
@@ -200,6 +204,10 @@ export default class PocomathInstance {
     * the corresponding changes to the _typed object immediately
     */
    installType(type, spec) {
+      if (this._templateParam(type)) {
+         throw new SyntaxError(
+            `Type name '${type}' reserved for template parameter`)
+      }
       if (type in this.Types) {
          if (spec !== this.Types[type]) {
             throw new SyntaxError(`Conflicting definitions of type ${type}`)
@@ -295,10 +303,13 @@ export default class PocomathInstance {
                opImps[signature] = behavior
                for (const dep of behavior.uses) {
                   const depname = dep.split('(', 1)[0]
-                  if (depname === 'self') continue
+                  if (depname === 'self' || this._templateParam(depname)) {
+                     continue
+                  }
                   this._addAffect(depname, name)
                }
                for (const type of typesOfSignature(signature)) {
+                  if (this._templateParam(type)) continue
                   this._usedTypes.add(type)
                   this._addAffect(':' + type, name)
                }
@@ -307,6 +318,12 @@ export default class PocomathInstance {
       }
    }
 
+   /* returns a boolean indicating whether t denotes a template parameter.
+    * We will start this out very simply: the special string `T` is always
+    * a template parameter, and that's the only one
+    */
+   _templateParam(t) { return t === 'T' }
+
    _addAffect(dependency, dependent) {
       if (dependency in this._affects) {
          this._affects[dependency].add(dependent)
@@ -366,7 +383,9 @@ export default class PocomathInstance {
       }
       Object.defineProperty(this, name, {configurable: true, value: 'limbo'})
       const tf_imps = {}
-      for (const [signature, {uses, does}] of usableEntries) {
+      for (const [rawSignature, {uses, does}] of usableEntries) {
+         /* For now, replace 'T' with 'any' */
+         const signature = rawSignature.replaceAll('T', 'any')
          if (uses.length === 0) {
             tf_imps[signature] = does()
          } else {
@@ -374,7 +393,10 @@ export default class PocomathInstance {
             let full_self_referential = false
             let part_self_references = []
             for (const dep of uses) {
-               const [func, needsig] = dep.split(/[()]/)
+               let [func, needsig] = dep.split(/[()]/)
+               const needTypes = needsig ? typesOfSignature(needsig) : new Set()
+               /* For now, punt on template parameters */
+               if (needTypes.has('T')) needsig = ''
                if (func === 'self') {
                   if (needsig) {
                      if (full_self_referential) {

src/generic/subtract.mjs

@@ -1,3 +1,3 @@
 export const subtract = {
-   'any,any': ({add, negate}) => (x,y) => add(x, negate(y))
+   'T,T': ({'add(T)': addT, 'negate(T)': negT}) => (x,y) => addT(x, negT(y))
 }