Add examples for custom (post)circumfix operators

Author: moritz

lib/Language/functions.pod

@@ -277,9 +277,36 @@ sub postfix:<!>(Int $x where { $x >= 0 }) { [*] 1..$x };
 say 6!;                     # 720
 =end code
 
-=begin comment
-TODO: elaborate on (post)circumfix ops
-=end comment
+The operator declaration becomes available as soon as possible, so you can
+even recurse into a just-defined operator, if you really want to:
+
+=begin code
+sub postfix:<!>(Int $x where { $x >= 0 }) {
+    $x == 0 ?? 1 !! $x * ($x - 1)!
+}
+say 6!;                     # 720
+=end code
+
+Circumfix and postcircumfix operators are made of two delimiters, one opening
+and one closing.
+
+=begin code
+sub circumfix:<START END>(*@elems) {
+    "start", @elems, "end"
+}
+
+say START 'a', 'b', 'c' END;        # start a b c end
+=end code;
+
+Postcircumfixes also receive the the term after which they are parsed as
+an argument:
+
+=begin code
+sub postcircumfix:<!! !!>($left, $inside) {
+    "$left -> ( $inside )"
+}
+say 42!! 1 !!;      # 42 -> ( 1 )
+=end code
 
 =head2 Precedence