Implementation restriction:
A union (with more than one term) cannot contain the
<a href="#Predeclared_identifiers">predeclared identifier</a> <code>comparable</code>
or interfaces that specify methods, or embed <code>comparable</code> or interfaces
that specify methods.
</p>

<p>
Interfaces that are not <a href="#Basic_interfaces">basic</a> may only be used as type
constraints, or as elements of other interfaces used as constraints.

MethodSpec         = MethodName Signature .
MethodName         = identifier .
InterfaceTypeName  = TypeName .
</pre>

<p>
An interface type may specify methods <i>explicitly</i> through method specifications,
or it may <i>embed</i> methods of other interfaces through interface type names.
</p>

<pre>
// A simple File interface.
interface {