feat: Start adding vector arithmetic

So far, abs, add, norm, normsq, and sum are supported. To get them
  to work, also implements the following:
  * refactor: Use ReturnType function rather than just accessing .returns
  * feat: distinguish marking a function as a behavior from its return type
  * refactor: Rename `NotAType` to `Unknown` because it must be made closer
    to a bona fide type for the sake of inhomogeneous vectors
  * feat: make resolving a TypeDispatcher method on a type vector including
    `Unknown` into a no-op; that simplifies a number of generic behaviors
  * feat: add `haszero` method parallel to `hasnan`
  * feat: track the Vector nesting depth of Vector specializations

src/vector/arithmetic.js

@@ -0,0 +1,33 @@
+import {promoteBinary, promoteUnary} from './helpers.js'
+import {Vector} from './Vector.js'
+
+import {ReturnType} from '#core/Type.js'
+import {match} from '#core/TypePatterns.js'
+import {ReturnsAs} from '#generic/helpers.js'
+
+export const normsq = match(Vector, (math, V) => {
+   const compNormsq = math.normsq.resolve(V.Component)
+   const sum = math.sum.resolve(Vector(ReturnType(compNormsq)))
+   return ReturnsAs(sum, v => sum(v.map(compNormsq)))
+})
+// abs and norm differ only on Vector (and perhaps other collections) --
+// norm computes overall by the generic formula, whereas abs distributes
+// elementwise:
+export const abs = promoteUnary('abs')
+export const add = promoteBinary('add')
+
+export const sum = match(Vector, (math, V) => {
+   const add = math.add.resolve([V.Component, V.Component])
+   const haszero = math.haszero(V.Component)
+   const zero = haszero ? math.zero(V.Component) : undefined
+   return ReturnsAs(add, v => {
+      if (v.length === 0) {
+         if (haszero) return zero
+         throw new TypeError(`Can't sum empty ${V}: no zero element`)
+      }
+      let ix = 0
+      let retval = v[ix]
+      while (++ix < v.length) retval = add(retval, v[ix])
+      return retval
+   })
+})