where
-- Allowed characters, taken from ForSyde's mkVHDLExtId
allowed = ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ " \"#&'()*+,./:;<=>_|!$%@?[]^`{}~-"
strip_invalid = filter (`elem` allowed)
where
-- Allowed characters, taken from ForSyde's mkVHDLExtId
allowed = ['A'..'Z'] ++ ['a'..'z'] ++ ['0'..'9'] ++ " \"#&'()*+,./:;<=>_|!$%@?[]^`{}~-"
strip_invalid = filter (`elem` allowed)