diff --git a/crsn/src/builtin/parse.rs b/crsn/src/builtin/parse.rs
index 75b231d..6880f48 100644
--- a/crsn/src/builtin/parse.rs
+++ b/crsn/src/builtin/parse.rs
@@ -71,6 +71,16 @@ impl CrsnExtension for BuiltinOps {
                 return Ok(ParseRes::ParsedNone);
             }
 
+            "unsym" => {
+                let alias = parse_reg_alias(args.next())?;
+
+                let mut pstate = pcx.state.borrow_mut();
+                if pstate.reg_aliases.remove(&alias).is_none() {
+                    return Err(CrsnError::Parse(format!("Register alias \"{}\" not defined!", alias).into()));
+                }
+                return Ok(ParseRes::ParsedNone);
+            }
+
             "def" => {
                 let name = parse_constant_name(args.next())?;
                 let value = parse_value(args.next())?;
@@ -89,6 +99,16 @@ impl CrsnExtension for BuiltinOps {
                 return Ok(ParseRes::ParsedNone);
             }
 
+            "undef" => {
+                let name = parse_constant_name(args.next())?;
+
+                let mut pstate = pcx.state.borrow_mut();
+                if pstate.constants.remove(&name).is_none() {
+                    return Err(CrsnError::Parse(format!("Constant \"{}\" not defined!", name).into()));
+                }
+                return Ok(ParseRes::ParsedNone);
+            }
+
             "j" => {
                 let dest = parse_label(args.next())?;
                 BuiltinOp::Jump(dest)