diff --git a/main.c b/main.c
index bfe1c82..901e039 100644
--- a/main.c
+++ b/main.c
@@ -337,31 +337,36 @@ struct Object *parens_part()
 
 struct Object *eval(struct Object *const program)
 {
+    // NULL is an empty list, can't eval
     if (!program) {
         error("can't eval null");
         return NULL;
     }
 
+    // Almost everything evaluates to itself
     if (program->type != TYPE_PAIR && program->type != TYPE_ATOM) {
         return program;
     }
 
+    // Atoms are variable names, but we can't lookup
     if (program->type == TYPE_ATOM) {
         error("can't eval atoms");
         return NULL;
     }
 
+    // The first item of pair should be an atom - a function name
     if (!program->pair.a || program->pair.a->type != TYPE_ATOM) {
         error("eval expects atom");
         return NULL;
     }
 
-    if (strcmp(program->pair.a->s, "sum") != 0) {
-        error("unknown func");
-        return NULL;
+    // The func "sum"
+    if (strcmp(program->pair.a->s, "sum") == 0) {
+        return func_sum(program->pair.b);
     }
 
-    return func_sum(program->pair.b);
+    error("unknown func");
+    return NULL;
 }
 
 struct Object *func_sum(struct Object *const numbers)