diff --git a/flake.nix b/flake.nix
index e36c155..9902215 100644
--- a/flake.nix
+++ b/flake.nix
@@ -45,6 +45,7 @@
                 clang-tools # clangd
                 gdb
                 gdbgui
+                rust-analyzer
               ];
             };
           };
diff --git a/runtime/.envrc b/runtime/.envrc
deleted file mode 100644
index 3550a30..0000000
--- a/runtime/.envrc
+++ /dev/null
@@ -1 +0,0 @@
-use flake
diff --git a/runtime/src/lib.rs b/runtime/src/lib.rs
index 17399c3..412631f 100644
--- a/runtime/src/lib.rs
+++ b/runtime/src/lib.rs
@@ -1,7 +1,14 @@
 use std::{io::{stdout, Write}};
 
-const scm_tc3_cons: u64 = 0;
-const scm_tc2_int: u64 = 2;
+const scm_tc2_int      : u64 = 2;
+const scm_tc3_cons     : u64 = 0;
+const scm_tc7_weak_set : u64 = 0x55;
+const scm_tc7_symbol   : u64 = 0x05;
+const scm_tc7_string   : u64 = 0x15;
+
+const scm_false : SCM = scm_pack (0b00100);
+const scm_true  : SCM = scm_pack (0b01100);
+const scm_eol   : SCM = scm_pack (0b10100);
 
 
 type scm_bits = u64;
@@ -12,7 +19,7 @@ struct SCM {
     n : scm_bits
 }
 
-fn scm_pack (bits : scm_bits) -> SCM {
+const fn scm_pack (bits : scm_bits) -> SCM {
     SCM {n: bits}
 }
 
@@ -88,9 +95,9 @@ pub extern "C" fn scm_write (x: SCM) -> SCM {
 }
 
 #[unsafe(no_mangle)]
-pub extern "C" fn scm_from_utf8_string (s: scm_bits, len: scm_bits) -> scm_bits {
+pub extern "C" fn scm_from_utf8 (s: &str) -> SCM {
     println! ("scm_from_utf8_string");
-    return 0;
+    return scm_pack (0);
 }
 
 pub fn add (left: u64, right: u64) -> u64 {
diff --git a/runtime/src/weak-set.rs b/runtime/src/weak-set.rs
new file mode 100644
index 0000000..551e803
--- /dev/null
+++ b/runtime/src/weak-set.rs
@@ -0,0 +1,2 @@
+use std::collections;
+