add Matrix example

FossilOrigin-Name: dbd9a4588a272ee38d70897ca36ad3698ef6e63df28af91e17e4b76f4211b9f5

example/Matrix.forth

@@ -0,0 +1,35 @@
+#!/usr/bin/env retro -i -s
+
+By WilhelmVonWeiner: Matrices that store their bounds and a
+couple rushedly written tests.
+
+~~~
+{{
+  :prepare (nms--knms) push dup-pair * rot rot pop ;
+  :create (nms--) d:create dup-pair * #2 + allot ;
+  :initialise (nm--) d:last<xt> store-next store-next ;
+  :fill (na--a)  [ store-next ] times ;
+---reveal---
+  :matrix (nms--a) create initialise ;
+  :matrix<xs> (x1xn...nms--a) prepare matrix swap fill drop ;
+}}
+~~~
+
+Test matrix, should print "matrix works!" and not "broken!".
+
+```
+:get-next n:dec dup fetch ;
+:broken 'broken s:put nl ;
+#3 #1 'tester matrix
+get-next #3 -eq? [ broken ] if
+get-next #1 -eq? [ broken ] if
+'matrix_works!_at_ s:put n:put nl
+```
+
+Test matrix<xs>, should be "contained!" thrice.
+
+```
+#30 #20 #10 #3 #1 'tester s:keep matrix<xs>
+[ tester #2 + #3 [ fetch-next swap ] times drop ] set:from-results
+[ { #10 #20 #30 } set:contains? [ 'contained! s:put sp nl ] if ] set:for-each
+```