Created jdaNotes.md

Added notes on what I've done to the code.

Author: johnallsup

src/JdaNotes.md

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+# Notes on Additions to PhpSets by John Allsup
+
+My webhost at present is a basic package with Ionos. 
+There is PHP, MySQL and Apache, but you get what you get, and can't, for exmaple, install things via PECL.
+So sets need to be implemented in PHP using arrays behind the scenes, and this is what this PhpSets does.
+My modifications are generally the result of 'scratching an itch' -- if it doesn't do something I want, and
+can easily be modified to do so, I hack in some code to do it. Thus I share things in a fork.
+
+-- John
+
+## 28th March, 2023
+
+### New Set From Array
+The upstream version only allowed instatiating sets by passing elements as arguments, rather than all elements in a single array.
+That is, if you have
+```php
+$elements = [1,2,3];
+```
+there was no way without using reflection to create a set with those elements. Now you can say
+```php
+$set = Set::FromArray($elements);
+```
+though note that, behind the scenes, this just uses reflection. At some point I'll take a deeper look into how it works
+and remove the dependency on reflection.
+
+### Family Union and Intersection
+Given a set A of sets { a_1, a_2, ..., a_n } the union UA is the collection of things that are members of *at least one* of the a_i.
+Similary the intersection of A is the collection of things that are in *every* one of the a_i. The vacuous case of union is the empty set.
+But in Set Theory as it appears in modern mathematics, the intersection if the empty set is the 'set of all sets'. (The reason
+is that 'X is in every member of A' is true for every X in the case where A is empty, since there is no possibility of a counterexample.
+For some X *not* to be in the intersection of A, there needs to be *some* B in A such that X is not in B.)
+
+What is there at present (2023-03-28) is correct, with the exception that the intersection of
+an empty family of sets results in the empty set, which is not how things work in actual Set Theory -- see
+for example [this math.stackexchange question](https://math.stackexchange.com/questions/959201/the-intersection-of-an-empty-family-of-sets).
+It is not efficient as is. It simply iteratively calls the `Set::intersect($set)` method.