Hello, is there syntax available for creating function based indexes?

How can I write this in s-sql?

,---- | CREATE INDEX my_index ON my_table (LOWER(my_column)) `----

I tried

,---- | (sql (:create-index :my-index :on :my-table :fields (:lower :my-column))) | | => (:LOWER :TERM) cannot be coerced to a string. | [Condition of type SIMPLE-TYPE-ERROR] `----

Thanks in advance, Richard