Fold operation (code/docs/test)

lexer.l

@@ -49,6 +49,7 @@ struct lexer_param;
 "and" { return AND; }
 "or" { return OR; }
 "end" { return END; }
+"fold" { return FOLD; }
 "//" { return DEFINEDOR; }
 "|=" { return SETPIPE; }
 "+=" { return SETPLUS; }