diff --git a/code/code_gen/code_gen.py b/code/code_gen/code_gen.py
index 5a10cf1..f2abc32 100644
--- a/code/code_gen/code_gen.py
+++ b/code/code_gen/code_gen.py
@@ -51,6 +51,7 @@ calc_grammar = """
              | "sqrt" -> sqrt
              | "abs" -> abs
              | "rxn_rate" -> rxn_rate
+             | "$" NAME   -> udf
 
     ?derivative: "ddx"  -> ddx
                | "d2dx" -> d2dx
diff --git a/code/code_gen/post.py b/code/code_gen/post.py
index def389a..cc08100 100644
--- a/code/code_gen/post.py
+++ b/code/code_gen/post.py
@@ -135,6 +135,7 @@ class ExpToCode(Transformer):
     sqrt = lambda self : "sqrt"
     abs = lambda self : "dabs"
     rxn_rate = lambda self : "rxn_rate"
+    udf = lambda self, a  : a.value