diff --git a/src/main.rs b/src/main.rs
index d49a190..6f3a637 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,6 +1,8 @@
+use std::collections::HashMap;
+
 struct VM<'p> { 
     program: &'p Program, 
-    block: u32, ip: u32, 
+    block: Block, ip: u32,
     heap: Heap<'p>,
     locals: Object,
     stack: Vec<Value>, 
@@ -35,11 +37,12 @@ impl Value {
 #[derive(Clone, Copy, Debug)]
 struct Object { index: u32 }  
 struct Program { 
-    blocks: Vec<BlockData>,
-    types: Vec<TypeData>,
+    blocks: Interner<BlockData>,
+    types: Interner<TypeData>,
 }
 struct BlockData { instructions: Vec<Instruction> }
 
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
 struct Block { index: u32 }
 
 #[derive(Debug)]
@@ -61,7 +64,7 @@ enum Instruction {
     Op21(Op21), // [t1, t2] -> [t3]
 
     // assert that the stack has n items and jump to the given block
-    Call { block: u32, n: u32, },  
+    Call { block: Block, n: u32, },  
     // assert that the stack has n items, clear it, and execute a syscall
     Syscall { syscall: u32, n: u32 },
 
@@ -78,6 +81,7 @@ enum Op11 { Get(u32) }
 #[derive(Debug)]
 enum Op21 { Add, Set(u32) }
 
+const BL_MAIN: Block = Block { index: 0x0 };
 const TY_NULL: Type = Type { id: 0x0 };
 const NULL: Object = Object { index: 0 };
 
@@ -85,9 +89,19 @@ const SYSCALL_DBG: u32 = 0x1000;
 
 
 fn main() {
+    _main().unwrap()
+}
+fn _main() -> Result<(), String> {
     let mut program = Program::new();
-    let ty_locals = program.type_create(3);
-    program.block_create(
+    let ty_locals = program.type_prototype("main.locals");
+    program.type_instantiate(ty_locals, 3)?;
+
+    let bl_main_0 = program.block_prototype("main");
+    let bl_main_1 = program.block_prototype("main.1");
+    let bl_main_2 = program.block_prototype("main.2");
+
+    program.block_instantiate(
+        bl_main_0,
         BlockData { 
             instructions: vec![ 
                 Instruction::PushNew(ty_locals), Instruction::PopLocals, 
@@ -97,12 +111,13 @@ fn main() {
                 Instruction::PushInteger(0x0000ba75), Instruction::Op21(Op21::Set(1)),
                 // stack: [locals]
 
-                Instruction::Call { block: 1, n: 1 },
+                Instruction::Call { block: bl_main_1, n: 1 },
             ]
         }
-    );
+    )?;
 
-    program.block_create(
+    program.block_instantiate(
+        bl_main_1,
         BlockData {
             instructions: vec![
                 Instruction::PopLocals,
@@ -113,26 +128,27 @@ fn main() {
                 Instruction::Op21(Op21::Add),
                 Instruction::Op21(Op21::Set(2)),
 
-                Instruction::Call { block: 2, n: 1 }
+                Instruction::Call { block: bl_main_2, n: 1 }
             ]
         }
-    );
+    )?;
 
-    program.block_create(
+    program.block_instantiate(
+        bl_main_2,
         BlockData { 
             instructions: vec![
                 Instruction::PopLocals,
 
                 Instruction::PushLocals,
-                Instruction::Op11(Op11::Get(0)),
+                Instruction::Op11(Op11::Get(2)),
                 Instruction::Syscall { syscall: SYSCALL_DBG, n: 1 },
 
                 Instruction::Exit,
             ]
         }
-    );
+    )?;
 
-    host_program(&program).unwrap()
+    host_program(&program)
 }
 
 fn host_program(p: &Program) -> Result<(), String> {
@@ -164,7 +180,7 @@ impl<'p> VM<'p> {
 
         VM {
             program,
-            block: 0,
+            block: BL_MAIN,
             ip: 0,
             heap,
             locals: NULL,
@@ -186,7 +202,7 @@ impl<'p> VM<'p> {
 
         let instruction = 
             self.program
-                .blocks.get(self.block as usize).ok_or("block must exist")?
+                .block_get(self.block)?
                 .instructions.get(self.ip as usize).ok_or("instruction must exist")?;
         self.ip += 1;
         println!("instruction: {:?}", instruction);
@@ -201,7 +217,7 @@ impl<'p> VM<'p> {
             }
 
             &Instruction::PushNew(ty) => {
-                let t = self.heap.object_create(ty);
+                let t = self.heap.object_create(ty)?;
                 self.stack.push(Value::Object(t))
             }
 
@@ -268,7 +284,7 @@ impl Op11 {
         match self {
             &Op11::Get(ix) => {
                 let t0 = v0.object().ok_or("must be a object")?;
-                Ok(heap.object_get_arg(t0, ix))
+                heap.object_get_arg(t0, ix)
             }
        }
     }
@@ -285,7 +301,7 @@ impl Op21 {
             }
             &Op21::Set(ix) => {
                 let t0 = v0.object().ok_or("must be a object")?;
-                heap.object_set_arg(t0, ix, v1);
+                heap.object_set_arg(t0, ix, v1)?;
                 Ok(v0)
             }
         }
@@ -296,28 +312,43 @@ impl Op21 {
 impl Program {
     fn new() -> Program {
         let mut p = Self {
-            blocks: vec![],
-            types: vec![],
+            blocks: Interner::<BlockData>::new(),
+            types: Interner::<TypeData>::new(),
         };
-        let ty_null = p.type_create(0);
+
+        let ty_null = p.type_prototype("null");
         assert_eq!(TY_NULL, ty_null);
+        p.type_instantiate(ty_null, 0).expect("cannot fail to create null instance");
+
+        let bl_main = p.block_prototype("main");
+        assert_eq!(BL_MAIN, bl_main);
+
+
         return p
     }
 
-    fn type_create(&mut self, len: u32) -> Type {
-        let index = self.types.len();
-        self.types.push(TypeData { len });
-        Type { id: index as u32 }
+    fn type_prototype(&mut self, name: &str) -> Type {
+        Type { id: self.types.prototype(name) }
     }
 
-    fn type_get(&self, ty: Type) -> &TypeData {
-        return self.types.get(ty.id as usize).expect("type should have existed")
+    fn type_instantiate(&mut self, ty: Type, len: u32) -> Result<(), &'static str> {
+        self.types.instantiate(ty.id, TypeData { len })
     }
 
-    fn block_create(&mut self, block: BlockData) -> Block {
-        let index = self.blocks.len();
-        self.blocks.push(block);
-        Block { index: index as u32 }
+    fn type_get(&self, ty: Type) -> Result<&TypeData, &'static str> {
+        self.types.get(ty.id)
+    }
+
+    fn block_prototype(&mut self, name: &str) -> Block {
+        Block { index: self.blocks.prototype(name) }
+    }
+
+    fn block_instantiate(&mut self, bl: Block, data: BlockData) -> Result<(), &'static str> {
+        self.blocks.instantiate(bl.index, data)
+    }
+
+    fn block_get(&self, bl: Block) -> Result<&BlockData, &'static str> {
+        self.blocks.get(bl.index)
     }
 }
 
@@ -347,54 +378,108 @@ impl<'p> Heap<'p> {
             program: p,
             data: vec![],
         };
-        let null = h.object_create(TY_NULL);
+        let null = h.object_create(TY_NULL).expect("shouldn't fail during Heap construction");
         assert_eq!(NULL.index, null.index);
         h
     }
 
-    fn object_create(&mut self, ty: Type) -> Object {
-        let td = self.program.type_get(ty);
+    fn object_create(&mut self, ty: Type) -> Result<Object, &'static str> {
+        let td = self.program.type_get(ty)?;
         let index = self.data.len() as u32;
         self.data.push(HeapItem::Type(ty));
         for _ in 0..td.len {
             self.data.push(HeapItem::Value(Value::Object(NULL)));
         }
-        Object { index }
+        Ok(Object { index })
     }
 
-    fn object_get_type(&self, object: Object) -> Type {
-        let header = self.data.get(object.index as usize).expect("object should have been valid");
+    fn object_get_type(&self, object: Object) -> Result<Type, &'static str> {
+        let header = self.data.get(object.index as usize).ok_or("object should have been valid")?;
         if let &HeapItem::Type(ty) = header {
-            ty
+            Ok(ty)
         } else {
-            panic!("header is invalid: {:?}", header)
+            Err("header is invalid")
         }
     }
 
-    fn object_get_arg(&self, object: Object, ix: u32) -> Value {
-        let ty = self.object_get_type(object);
-        let td = self.program.type_get(ty);
+    fn object_get_arg(&self, object: Object, ix: u32) -> Result<Value, &'static str> {
+        let ty = self.object_get_type(object)?;
+        let td = self.program.type_get(ty)?;
 
         if ix < td.len {
             let value = self.data.get((object.index + ix + 1) as usize).expect("should have been present");
             match value {
-                &HeapItem::Value(si) => si,
-                _ => panic!("should have been a Value")
+                &HeapItem::Value(si) => Ok(si),
+                _ => return Err("should have been a Value"),
             }
         } else {
-            panic!("invalid ix: {}", ix)
+            return Err("invalid field index")
         }
     }
 
-    fn object_set_arg(&mut self, object: Object, ix: u32, arg: Value) {
-        let ty = self.object_get_type(object);
-        let td = self.program.type_get(ty);
+    fn object_set_arg(&mut self, object: Object, ix: u32, arg: Value) -> Result<(), &'static str> {
+        let ty = self.object_get_type(object)?;
+        let td = self.program.type_get(ty)?;
 
         if ix < td.len {
             let slot = self.data.get_mut((object.index + ix + 1) as usize).expect("should have been present");
-            *slot = HeapItem::Value(arg)
+            *slot = HeapItem::Value(arg);
+            Ok(())
         } else {
-            panic!("invalid ix: {}", ix)
+            return Err("invalid field index")
         }
     }
-}
\ No newline at end of file
+}
+
+struct Interner<T> {
+    by_name: HashMap<String, u32>,
+    names: Vec<String>,
+    values: Vec<Option<T>>
+}
+
+impl<T> Interner<T> {
+    pub fn new() -> Self {
+        Self {
+            by_name: HashMap::new(),
+            names: vec![],
+            values: vec![],
+        }
+    }
+
+    pub fn prototype(&mut self, name: &str) -> u32 {
+        if let Some(v) = self.by_name.get(name) { return *v; }
+
+        let ix = self.values.len() as u32;
+        self.by_name.insert(name.to_string(), ix);
+        self.names.push(name.to_string());
+        self.values.push(None);
+        ix
+    }
+
+    pub fn instantiate(&mut self, handle: u32, value: T) -> Result<(), &'static str> {
+        let slot = self.values
+            .get_mut(handle as usize)
+            .ok_or("handle was not prototyped on this interner")?;
+        if slot.is_some() {
+            return Err("can't instantiate handle twice")
+        }
+        *slot = Some(value);
+        Ok(())
+    }
+
+    pub fn get(&self, handle: u32) -> Result<&T, &'static str> {
+        match self.values.get(handle as usize) {
+            Some(Some(h)) => Ok(h),
+            Some(_) => Err("handle was prototyped but not instantiated"),
+            _ => Err("handle was not prototyped on this interner"),
+        }
+    }
+
+    pub fn get_mut(&mut self, handle: u32) -> Result<&mut T, &'static str> {
+        match self.values.get_mut(handle as usize) {
+            Some(Some(h)) => Ok(h),
+            Some(_) => Err("handle was prototyped but not instantiated"),
+            _ => Err("handle was not prototyped on this interner"),
+        }
+    }
+}