diff --git a/lectures/lambda.md b/lectures/lambda.md
index f77228f..f17db38 100644
--- a/lectures/lambda.md
+++ b/lectures/lambda.md
@@ -9,8 +9,7 @@ The **Church-Turing Thesis** states that the functions defined by lambda calculu
 
 ## The Grammar of Lambda
 ```
-Lambda ::= Variable | Group | Abstraction | Application;
-Variable ::= <letter> 
+Lambda ::= <variable> | Group | Abstraction | Application;
 Group ::= '(' Lambda ')';
 Abstraction ::= 'λ' <variable> '.' Lambda;
 Application ::= Lambda Lambda