Keyword is unavailable.
          #2052   Compile-time error
          Keyword is unavailable.
Additional Information
 Keyword 'X' has been overridden by a function definition.
     or
 Keyword 'X' has been overridden by a variable definition.
     or
 Keyword 'X' has been overridden by an opaque type declaration.
Explanation
Put a backquote at the start of the keyword to indicate that you want
the keyword and not the function or variable of the same name.