diff --git a/6.0s1.h b/6.0s1.h
index fe1ff54..597d5fc 100644
--- a/6.0s1.h
+++ b/6.0s1.h
@@ -27,6 +27,12 @@ void s1_assert(int value, const char *reason, struct s1_location loc) {
     }
 }
 
+void s1_trace(const char *text, struct s1_location loc) {
+    fprintf(stderr, "Trace: %s:%d: %s\n", loc.file, loc.line, text);
+}
+
+
+#define s1_trace(text) s1_trace(text, S1_LOC())
 
 // 2. simple IO
 
@@ -41,7 +47,7 @@ void write_long(S1_LONG value, struct s1_location loc) {
 }
 
 void write_double(S1_DOUBLE value, struct s1_location loc) {
-    int rc = printf("%f", value);
+    int rc = printf("%lf", value);
     s1_assert(rc != EOF, "Error: write_long", loc);
 }