(def foo-system [w dt]
  (doseq [eid (world/query w (all :FooComponent
                                  (any :BarComponent
                                       :BazComponent)))]
    (world/remove-component w eid :FooComponent)))

(fact "foo-system does stuff."
      (let [w (mock/mock-editable-world)
            q (all :FooComponent
                   (any :BarComponent
                        :BazComponent))]
        (foo-system w anything) => anything
        (provided (mock/query w q) => [..e1.. ..e2..]
                  (mock/remove-component w ..e1.. :FooComponent) => nil
                  (mock/remove-component w ..e2.. :FooComponent) => nil)))

Mock worlds that delegate their operations to functions.

Because protocol method can't be mocked using `with-redefs`
etc. systems and world backends are difficult to unit test.
Paying one level of indirection, protocols can be mocked
using functions defined in this module.

#### Examples:

    (def foo-system [w dt]
      (doseq [eid (world/query w (all :FooComponent
                                      (any :BarComponent
                                           :BazComponent)))]
        (world/remove-component w eid :FooComponent)))

    (fact "foo-system does stuff."
          (let [w (mock/mock-editable-world)
                q (all :FooComponent
                       (any :BarComponent
                            :BazComponent))]
            (foo-system w anything) => anything
            (provided (mock/query w q) => [..e1.. ..e2..]
                      (mock/remove-component w ..e1.. :FooComponent) => nil
                      (mock/remove-component w ..e2.. :FooComponent) => nil)))