Adds set algebra to the Math page

Which might or might not be what someone 1000 days ago had in mind,
but it's at least what I would like to find if I go looking for math
stuff in Perl 6. This refs #114, but still a lot TBD.

Author: JJ

doc/Language/math.pod6

@@ -46,6 +46,25 @@ say "Identity with Ø ", so @id-empty.all;       # OUTPUT: «Identity with Ø Tr
 
 In this code, not only we check if the equalities in the algebra of sets hold, we also use, via L<sigilless variables|/language/variables#index-entry-\_(sigilless_variables)> and the Unicode form of the set operators, expressions that are as close as possible to the original form; C<A ∪ U === U>, for example, except for the use of the L<value identity operator <===>|/routine/===> is very close to the actual mathematical expression in the L<Wikipedia entry|https://en.wikipedia.org/wiki/Algebra_of_sets>.
 
+We can even test de Morgan's law, as in the code below:
+
+=begin code
+my @alphabet = 'a'..'z';
+my \U = @alphabet.Set;
+sub postfix:<⁻>(Set $a) { U ⊖ $a }
+my @sets;
+@sets.push: Set.new( @alphabet.pick( @alphabet.elems.rand)) for @alphabet;
+my ($de-Morgan1,$de-Morgan2) = (True,True);
+for @sets X @sets -> (\A, \B){
+    $de-Morgan1 &&= (A ∪ B)⁻  === A⁻ ∩ B⁻;
+    $de-Morgan2 &&= (A ∩ B)⁻  === A⁻ ∪ B⁻;
+}
+say "1st De Morgan is ", $de-Morgan1;
+say "2nd De Morgan is ", $de-Morgan2;
+=end code
+
+We declare C<⁻> as the I<complement> operation, which computes the symmetrical difference ⊖ between the Universal set C<U> and our set. Once that is declared, it is relatively easy to express operations such as the complementary of the union of A and B C<(A ∪ B)⁻>, with a notation that is very close to the original mathematical notation.
+
 =head1 Arithmetic.
 
 TBD

xt/words.pws

@@ -1403,3 +1403,4 @@ xFB
 xFC
 xFD
 dpkg
+equalities