Section 4.5 "General Category" defines a set of character categories.
Go treats all characters in any of the Letter categories Lu, Ll, Lt, Lm, or Lo
as Unicode letters, and those in the Number category Nd as Unicode digits.
</p>

<h3 id="Letters_and_digits">Letters and digits</h3>

<p>
The underscore character <code>_</code> (U+005F) is considered a letter.
</p>
<pre class="ebnf">
letter        = unicode_letter | "_" .

references to the virtual stack pointer from references to the architectural
<code>SP</code> register.
That is, <code>x-8(SP)</code> and <code>-8(SP)</code>
are different memory locations:
the first refers to the virtual stack pointer pseudo-register,
while the second refers to the
hardware's <code>SP</code> register.
</p>

<p>
On machines where <code>SP</code> and <code>PC</code> are
traditionally aliases for a physical, numbered register,

</p>

<pre>
type T1[P T1[P]] …                    // illegal: T1 refers to itself
type T2[P interface{ T2[int] }] …     // illegal: T2 refers to itself
type T3[P interface{ m(T3[int])}] …   // illegal: T3 refers to itself
type T4[P T5[P]] …                    // illegal: T4 refers to T5 and
type T5[P T4[P]] …                    //          T5 refers to T4