diff --git a/makefile b/makefile
index 43ff86b..d31bfd0 100644
--- a/makefile
+++ b/makefile
@@ -7,6 +7,9 @@
 
 ## C compiler
 CC=tcc # much faster compilation than gcc
+COMPILER_FLAGS=-g3 -Wall -Wextra -Wconversion -Wdouble-promotion -Wno-unused-parameter -Wno-unused-function -Wno-sign-conversion -fsanitize=undefined -fsanitize-trap
+## ^ from <https://nullprogram.com/blog/2023/04/29/>
+## <https://news.ycombinator.com/item?id=35758898>
 
 ## Debugging options
 DEBUG=-g#-g
@@ -31,7 +34,7 @@ STYLE_BLUEPRINT=webkit
 FORMATTER=clang-format -i -style=$(STYLE_BLUEPRINT)
 
 build: $(SRC)
-	$(CC)  -Wall  $(INCS) $(SRC) $(MPC) -o mumble $(LIBS)
+	$(CC) $(COMPILER_FLAGS)  $(INCS) $(SRC) $(MPC) -o mumble $(LIBS)
 
 format: $(SRC)
 	$(FORMATTER) $(SRC)
diff --git a/mumble b/mumble
index 0bb786f..7e0ed09 100755
Binary files a/mumble and b/mumble differ
diff --git a/notes/savepoint.txt b/notes/savepoint.txt
index 788b7f3..d82f295 100644
--- a/notes/savepoint.txt
+++ b/notes/savepoint.txt
@@ -1 +1,2 @@
 https://buildyourownlisp.com/chapter7_evaluation#trees
+https://buildyourownlisp.com/chapter9_s_expressions
diff --git a/src/mumble.c b/src/mumble.c
index 102d010..607f3f2 100644
--- a/src/mumble.c
+++ b/src/mumble.c
@@ -14,7 +14,7 @@ typedef struct {
   int err;
 } lispval;
 
-enum { LISPVAL_NUM, LISPVAL_ERR };
+enum { LISPVAL_NUM, LISPVAL_ERR, LISPVAL_SYM, LISPVAL_SEXPR };
 enum { LISPERR_DIV_ZERO, LISPERR_BAD_OP, LISPERR_BAD_NUM };
 
 lispval lispval_num(long x){