tt>/<tt>union</tt> can be dereferenced by a string key,
giving the field name. The field address is computed as the base
address plus the relative offset of the field. A read access loads the
field value and <a href="#convert_tolua">converts it to a Lua
object</a>. A write access <a href="#convert_fromlua">converts a Lua
object to the field type</a> and stores the converted value to the
field. An error is raised if a write access to a constant
<tt>struct</tt>/<tt>union</tt> or a constant field is attempted.