title: primStringEquality not reducing
body: <p>When asking Agda to normalize <code>test</code> on the following program:</p>
  
  <pre><code>data Bool : Set where
    T : Bool
    F : Bool
  {-# BUILTIN BOOL  Bool  #-}
  {-# BUILTIN TRUE  T #-}
  {-# BUILTIN FALSE F #-}
  
  postulate String : Set
  postulate primStringEquality : String → String → Bool
  {-# BUILTIN STRING String #-}
  
  test : Bool
  test = primStringEquality "bar" "foo"
  </code></pre>
  
  <p>It returns <code>primStringEquality "bar" "foo"</code> instead of <code>F</code>. Why?</p>