add quad quad* and quad@ as examples

FossilOrigin-Name: 9b50fa422adef2cc666629192d6acb81455506881cf4a665bf988f5ae336ea53

example/quad.forth

@@ -0,0 +1,52 @@
+The standard RETRO language provides `bi` and `tri` combinators to apply quotes to two or three values in various combinations. Sometimes it may be necessary to do this with four values.
+
+Note that this is *ugly* code. It's functional, but if you can refactor to avoid needing it, it'll likely be better in the long run.
+
+`quad` applies four quotes to a value. These are equivilent:
+
+  #10 [ #1 + ] call
+  #10 [ #1 - ] call
+  #10 [ #2 + ] call
+  #10 [ #3 + ] call
+
+  #10 [ #1 + ] [ #1 - ] [ #2 + ] [ #3 + ] quad
+
+~~~
+:quad (xqqqq-)
+  'abcde 'abacadae reorder
+  push push push push push push
+  call pop pop call pop pop call
+  pop pop call ;
+~~~
+
+`quad*` takes eight values (!) and applies each quote to a specific value. E.g., these are equivilent:
+
+  #10 [ #1 + ] call
+  #11 [ #1 - ] call
+  #12 [ #2 + ] call
+  #13 [ #3 + ] call
+
+  #10 #11 #12 #13 [ #1 + ] [ #1 - ] [ #2 + ] [ #3 + ] quad*
+
+~~~
+:quad* (abcdqqqq-)
+  'abcdefgh 'aebfcgdh reorder
+  push push push push push push
+  call pop pop call pop pop call
+  pop pop call ;
+~~~
+
+`quad@` takes four values and a quote, and applies the quote to each value in order. These are equivilent:
+
+  #10 [ #1 + ] call
+  #11 [ #1 + ] call
+  #12 [ #1 + ] call
+  #13 [ #1 + ] call
+
+  #10 #11 #12 #13 [ #1 + ] quad@
+
+~~~
+:quad@ (abcdq-)
+  'abcde 'abcdeeee reorder quad* ;
+~~~
+