Add a Boolean "not" operator.

Not wholly convinced this is a good idea, maybe "not" should be a
builtin function instead? 'not (.a == .b)' vs. '.a == .b | not'

c/parser.y

@@ -36,6 +36,7 @@
 %token END "end"
 %token AND "and"
 %token OR "or"
+%token NOT "not"
 %right "//"
 %nonassoc '=' SETPIPE
 %nonassoc EQ
@@ -111,6 +112,10 @@ Exp "and" Exp {
   $$ = gen_and($1, $3);
 } |
 
+"not" Exp {
+  $$ = gen_not($2);
+} |
+
 Exp "//" Exp {
   $$ = gen_definedor($1, $3);
 } |