interface type is <code>nil</code>.
</p>

<pre class="ebnf">
InterfaceType  = "interface" "{" { InterfaceElem ";" } "}" .
InterfaceElem  = MethodElem | TypeElem .
MethodElem     = MethodName Signature .
MethodName     = identifier .
TypeElem       = TypeTerm { "|" TypeTerm } .
TypeTerm       = Type | UnderlyingType .
UnderlyingType = "~" Type .
</pre>

<p>