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In the (very simplified) example below I want to check that all
product's names appear in the list of product names in a Store. 
However, I always get this error:
Error: In environment
p : Product
s : Store
The term "products s" has type "list Product"
while it is expected to have type "Product".

Any Ideas??

Example Code: 
Require Import String.
Require Import List.
Record Product:Set := {productName:string}.
Record Store:Set := {products:list Product}.

Theorem T: forall p:Product,forall s:Store, In (productName p) (productName (products s)).
(*A product's name must be in the list of Product Names that a Store has.
How is this written?  I don't want to check that the Product is in the list of 
Products, only that the name is in the Store's list of product names.*)