Documentation for Nothing

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 <p>The four roles (subject, function, two kinds of modifier) describe expressions, not values. When an expression is evaluated, the value's <a href="types.html">type</a> doesn't have to correspond to its role, and can even change from one evaluation to another. An expression's role is determined entirely by its source code, so it's fixed.</p>
+<p>In the table, <code><span class='Value'>?</span></code> marks an optional left argument. If there isn't a value in that position, or it's <a href="#nothing">Nothing</a> (<code><span class='Nothing'>·</span></code>), the middle function will be called with only one argument.</p>
 <p>If you're comfortable reading <a href="https://en.wikipedia.org/wiki/Backus%E2%80%93Naur_form">BNF</a> and want to understand things in more detail than described below, you might check the <a href="../spec/grammar.html">grammar specification</a> as well.</p>
 <h2 id="syntactic-role"><a class="header" href="#syntactic-role">Syntactic role</a></h2>
 <p><em>This issue is approached from a different angle in <a href="context.html">Context free grammar</a>.</em></p>
@@ -98,6 +99,15 @@
 <p>Variable names can be written in any case and with underscores added, and these changes don't affect what <a href="lexical.html">identifier</a> the name refers to. <code><span class='Value'>ab</span></code>, <code><span class='Value'>aB</span></code>, <code><span class='Function'>AB</span></code>, and <code><span class='Modifier2'>_a_B_</span></code> are all the same variable. However, the spelling—specifically the first and last characters—determine the variable's role. A lowercase first letter indicates a subject, and an uppercase first letter makes it a function. A leading underscore (regardless of the following character) indicates a 1-modifier, and both leading and trailing underscores makes a 2-modifier.</p>
 <p>Besides these, character, string, and <a href="arrayrepr.html#list-literals">list literals</a> always have a subject role, and the role of a <a href="block.html">block</a> is determined by its type, which depends either on the header it has or which special variables it uses.</p>
 <p>The role of a compound expression, formed by applying an operation to some inputs, depends on the operation applied. This system is discussed in the remaining sections below.</p>
+<h2 id="nothing"><a class="header" href="#nothing">Nothing</a></h2>
+<p>The character <code><span class='Nothing'>·</span></code> is called Nothing. While it can be easier to think of it as a value, it can't be passed around in variables, and so can also be interpreted as an element of syntax. The special name <code><span class='Value'>𝕨</span></code> also functions as Nothing if the block that contains it is called with one argument (the uppercase spelling <code><span class='Function'>𝕎</span></code> doesn't, but instead immediately causes an error). Both <code><span class='Nothing'>·</span></code> and <code><span class='Value'>𝕨</span></code> have a subject role.</p>
+<p>The following rules apply to Nothing:</p>
+<ul>
+<li>If it's the left argument in a function call, the function is called with no left argument.</li>
+<li>If it's the right argument, the function isn't called, and &quot;returns&quot; Nothing.</li>
+</ul>
+<p>For example, the expression <code><span class='Paren'>(</span><span class='Function'>F</span> <span class='Number'>2</span> <span class='Function'>G</span> <span class='Nothing'>·</span><span class='Paren'>)</span> <span class='Function'>H</span> <span class='Function'>I</span> <span class='Value'>j</span></code> is equivalent to <code><span class='Function'>H</span> <span class='Function'>I</span> <span class='Value'>j</span></code>. But functions and arguments that would be discarded by the second rule are still evaluated, so that for example <code><span class='Paren'>(</span><span class='Value'>a</span><span class='Function'>+</span><span class='Gets'>↩</span><span class='Number'>1</span><span class='Paren'>)</span> <span class='Function'>F</span> <span class='Nothing'>·</span></code> increments <code><span class='Value'>a</span></code> when run.</p>
+<p>Nothing can only be used as an argument to a function, or the left argument in a train (it can't be the right argument in a train because a train ends with a function by definition). In another position where a subject could appear, like as an operand or in a list, it causes an error: either at compile time, for <code><span class='Nothing'>·</span></code>, or when the function is called with no left argument, for <code><span class='Value'>𝕨</span></code>.</p>
 <h2 id="kinds-of-application"><a class="header" href="#kinds-of-application">Kinds of application</a></h2>
 <p>Here is a table of the modifier and function application rules:</p>
 <table>