Implement if else support

src/parser.rs

@@ -8,12 +8,12 @@ pub enum Datum {
     Symbol(String),
     List(Vec<Datum>),
     Procedure(fn(Vec<Datum>) -> Datum),
-    None,
+    Unspecified,
 }
 
 impl Default for Datum {
     fn default() -> Self {
-        Datum::None
+        Datum::Unspecified
     }
 }