docs: delete equivalence topic

(cherry picked from commit e8016a2303)

docs/language/learn-ql/advanced/advanced-ql.rst

@@ -13,5 +13,4 @@ Topics on advanced uses of QL. These topics assume that you are familiar with QL
 -  :doc:`Semantics of abstract classes <abstract-classes>`
 -  :doc:`Choosing appropriate ways to constrain types <constraining-types>`
 -  :doc:`Determining the most specific types of a variable <determining-specific-types-variables>`
--  :doc:`Understanding the difference between != and not(=) <equivalence>`
 -  :doc:`Monotonic aggregates in QL <monotonic-aggregates>`

docs/language/learn-ql/advanced/equivalence.rst

@@ -1,44 +0,0 @@
-Understanding the difference between != and not(=)
-==================================================
-
-The two expressions:
-
-#. ``a() != b()``
-#. ``not(a() = b())``
-
-look equivalent - so much so that inexperienced (and even experienced) programmers have been known to rewrite one as the other. However, they are not equivalent due to the quantifiers involved.
-
-Thinking of ``a()`` and ``b()`` as sets of values, the first expression says that there is a pair of values (one from each side of the inequality) which are different.
-
-**Using !=**
-
-::
-
-   exists x, y | x in a() and y in b() | x != y
-
-The second expression, however, says that it is *not* the case that there is a pair of values which are the *same* - that is, *all* pairs of values are different:
-
-**Using not(=)**
-
-::
-
-   not exists x, y | x in a() and y in b() | x = y
-
-This is equivalent to: ``forall x, y | x in a() and y in b() | x != y``. The meaning is very different from the first expression.
-
-Examples
---------
-
-``a() = {1, 2}`` and ``b() = {1}``:
-
-#. ``a() != b()`` is true, because ``2 != 1``
-#. ``a() = b()`` is true, because ``1 = 1``
-#. Therefore\ ``: not(a() = b())`` is false - a different answer from the comparison ``a() != b()``
-
-Similarly with ``a() = {}`` and ``b() = {1}``:
-
-#. ``a() != b()`` is false, because there is no value in ``a()`` that is not equal to ``1``
-#. ``a() = b()`` is also false, because there is no value in ``a()`` that is equal to ``1`` either
-#. Therefore: ``not(a() = b())`` is true - again a different answer from the comparison ``a() != b()``
-
-In summary, the QL expressions ``a() != b()`` and ``not(a() = b())`` may look similar, but their meaning is quite different.