diff --git a/kalk/src/symbol_table.rs b/kalk/src/symbol_table.rs
index 0d993cc..abd27ab 100644
--- a/kalk/src/symbol_table.rs
+++ b/kalk/src/symbol_table.rs
@@ -74,7 +74,9 @@ impl SymbolTable {
     }
 
     pub fn contains_fn(&self, identifier: &str) -> bool {
-        prelude::UNARY_FUNCS.contains_key(identifier)
+        identifier == "sum"
+            || identifier == "Σ"
+            || prelude::UNARY_FUNCS.contains_key(identifier)
             || prelude::BINARY_FUNCS.contains_key(identifier)
             || self.hashmap.contains_key(&format!("fn.{}", identifier))
     }