feat(poortf): add concept of 'author' to an imp of a TF

And make it so that if the same author adds imps again, they replace
  the previous imps from that author.

  Use this feature to avoid the explicit check for double-inclusion in
  subtract, and test it worked.

generic/subtract.js

@@ -1,9 +1,9 @@
 export default function create(pmath) {
    const add = pmath('add')
    const negate = pmath('negate')
-   if (!pmath.subtract) { // avoid double definition at cost of extensibility
-      pmath('subtract', [args => args.length === 2,
-         (x, y) => add(x, negate(y))])
-   }
+   return pmath(
+      'subtract',
+      [args => args.length === 2, (x, y) => add(x, negate(y))],
+      create)
    return pmath.subtract
 }