diff --git a/src/interpreter.rs b/src/interpreter.rs
index 237a694..14c12e6 100644
--- a/src/interpreter.rs
+++ b/src/interpreter.rs
@@ -1,15 +1,15 @@
 use crate::parser::Datum;
 use crate::parser::Datum::*;
-
-fn add(args: Vec<Datum>) -> Datum {
-    Number(42)
-}
+use crate::primitives::*;
+use std::mem;
 
 fn lookup_variable_value(exp: Datum) -> Datum {
     if let Symbol(string) = exp {
         match string.as_str() {
             "+" => Procedure(add),
-            _ => panic!("unbound-variable {:?}", string)
+            "*" => Procedure(mul),
+            "-" => Procedure(sub),
+            _ => panic!("unbound-variable {:?}", string),
         }
     } else {
         panic!("LOOKUP-VARIABLE-VALUE -- not-a-symbol {:?}", exp)
@@ -18,9 +18,19 @@ fn lookup_variable_value(exp: Datum) -> Datum {
 
 fn application(exp: Datum) -> Datum {
     if let List(vector) = exp {
-        vector[0]
+        if vector.len() == 0 {
+            panic!("Application needs at least a procedure")
+        }
+
+        let mut args: Vec<Datum> = vector.into_iter().map(interpret).collect();
+        if let Procedure(proc) = mem::take(&mut args[0]) {
+            // FIXME: call procedures with args only
+            proc(args)
+        } else {
+            panic!("The object {:?} is not applicable", args[0])
+        }
     } else {
-        panic!("APPLICATION -- cannot-apply {:?}", exp)
+        panic!("Cannot apply {:?}", exp)
     }
 }
 
@@ -29,6 +39,6 @@ pub fn interpret(exp: Datum) -> Datum {
         Boolean(_) | Number(_) => exp,
         Symbol(_) => lookup_variable_value(exp),
         List(_) => application(exp),
-        _ => panic!("unknown-expression {:?}", exp)
+        _ => panic!("unknown-expression {:?}", exp),
     }
-}
\ No newline at end of file
+}
diff --git a/src/lexer.rs b/src/lexer.rs
index ad2a3d0..a6cc661 100644
--- a/src/lexer.rs
+++ b/src/lexer.rs
@@ -70,12 +70,12 @@ fn scan_boolean(code: &str, ix: usize, mut tokens: Tokens) -> Tokens {
         "#t" => {
             let token = Token::Boolean(true);
             tokens.push(token);
-        },
+        }
         "#f" => {
             let token = Token::Boolean(false);
             tokens.push(token);
         }
-        _ => panic!("Expected boolean but got {}", c)
+        _ => panic!("Expected boolean but got {}", c),
     }
     return scan(code, ix + 2, tokens);
 }
@@ -135,6 +135,4 @@ const SPECIAL_INITIAL: &[char] = &[
     '!', '$', '%', '&', '*', '/', ':', '<', '=', '>', '?', '^', '_', '~', '+', '-',
 ];
 
-const SPECIAL_SUBSEQUENT: &[char] = &[
-    '+', '-', '.', '@',
-];
+const SPECIAL_SUBSEQUENT: &[char] = &['+', '-', '.', '@'];
diff --git a/src/main.rs b/src/main.rs
index 1bb50aa..f81a901 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -1,11 +1,11 @@
+mod interpreter;
 mod lexer;
 mod parser;
-mod interpreter;
-
+mod primitives;
 
 fn main() {
     // let scm_code = "(+ a (* 32 b) c #t #f)";
-    let scm_code = "+";
+    let scm_code = "(* 1 (- 2 2 2) 3)";
     let tokens = lexer::read(scm_code);
     let datum = parser::parse(tokens);
     let result = interpreter::interpret(datum);
diff --git a/src/parser.rs b/src/parser.rs
index 437b47c..dda21ed 100644
--- a/src/parser.rs
+++ b/src/parser.rs
@@ -1,8 +1,6 @@
 use crate::lexer::Token;
 use crate::lexer::Tokens;
 
-
-
 #[derive(Debug)]
 pub enum Datum {
     Boolean(bool),
@@ -10,6 +8,13 @@ pub enum Datum {
     Symbol(String),
     List(Vec<Datum>),
     Procedure(fn(Vec<Datum>) -> Datum),
+    None,
+}
+
+impl Default for Datum {
+    fn default() -> Self {
+        Datum::None
+    }
 }
 
 pub fn parse(tokens: Tokens) -> Datum {
@@ -23,14 +28,14 @@ fn parse_datum(tokens: &Tokens, ix: usize) -> (Datum, usize) {
         Token::Boolean(b) => (Datum::Boolean(*b), ix + 1),
         Token::Number(n) => (Datum::Number(*n), ix + 1),
         Token::LeftRoundBracket => parse_list(tokens, ix + 1),
-        _ => panic!("Unexpected token {:?}", tokens[ix])
+        _ => panic!("Unexpected token {:?}", tokens[ix]),
     }
 }
 
 pub fn parse_list(tokens: &Tokens, mut ix: usize) -> (Datum, usize) {
     let mut datums = vec![];
 
-    // FIXME: will crash when RightRoundBracket is missing 
+    // FIXME: will crash when RightRoundBracket is missing
     while tokens[ix] != Token::RightRoundBracket {
         let (datum, new_ix) = parse_datum(tokens, ix);
         datums.push(datum);
diff --git a/src/primitives.rs b/src/primitives.rs
new file mode 100644
index 0000000..4ba9a23
--- /dev/null
+++ b/src/primitives.rs
@@ -0,0 +1,58 @@
+use crate::parser::Datum;
+use crate::parser::Datum::*;
+use std::mem;
+
+pub fn add(mut args: Vec<Datum>) -> Datum {
+    let mut r = 0;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r += n;
+        } else {
+            panic!("ADD -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn mul(mut args: Vec<Datum>) -> Datum {
+    let mut r = 1;
+    for i in 1..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r *= n;
+        } else {
+            panic!("MUL -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}
+
+pub fn sub(mut args: Vec<Datum>) -> Datum {
+    if args.len() == 2 {
+        let datum = mem::take(&mut args[1]);
+        if let Number(n) = datum {
+            return Number(-n);
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+
+    let mut r;
+    let datum = mem::take(&mut args[1]);
+    if let Number(n) = datum {
+        r = n;
+    } else {
+        panic!("SUB -- not-all-args-are-numbers {:?}", args)
+    }
+
+    for i in 2..args.len() {
+        let datum = mem::take(&mut args[i]);
+        if let Number(n) = datum {
+            r -= n;
+        } else {
+            panic!("SUB -- not-all-args-are-numbers {:?}", args)
+        }
+    }
+    Number(r)
+}