1 files changed, 8 insertions, 3 deletions
diff --git a/docs/doc/undo.html b/docs/doc/undo.html
index daf1b60d..7e498365 100644
--- a/docs/doc/undo.html
+++ b/docs/doc/undo.html
@@ -21,7 +21,7 @@
 </pre>
 <p>Here it undoes a function to decrement the last character by incrementing that character. In part this is enabled by the clean design of BQN primitives, because better-behaved functions like those using structural <a href="under.html">Under</a> are easier to invert.</p>
 <h2 id="the-rules"><a class="header" href="#the-rules">The rules</a></h2>
-<p>If <code><span class='Function'>𝔽</span></code> can be inverted exactly, then Undo just does that. However, there are also some other functions that BQN inverts. For example, the squaring function <code><span class='Function'>×</span><span class='Modifier'>˜</span></code> has both a positive and a negative inverse, and yet:</p>
+<p>If <code><span class='Function'>𝔽</span></code> can be inverted exactly, then Undo just does that (or tries). However, there are also some other functions that BQN inverts. For example, the squaring function <code><span class='Function'>×</span><span class='Modifier'>˜</span></code> has both a positive and a negative inverse, and yet:</p>
 <a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=w5fLnCDCrzMKw5fLnOKBvCDDl8ucIMKvMyAgIyBJdCdzIG5vdCB0aGUgc2FtZSE=">↗️</a><pre>    <span class='Function'>×</span><span class='Modifier'>˜</span> <span class='Number'>¯3</span>
 9
     <span class='Function'>×</span><span class='Modifier'>˜⁼</span> <span class='Function'>×</span><span class='Modifier'>˜</span> <span class='Number'>¯3</span>  <span class='Comment'># It's not the same!
@@ -37,8 +37,9 @@
 <p>Arithmetic and simple combinators are usually invertible. A compound function that refers to its argument just once, like <code><span class='Number'>6</span><span class='Function'>+⌽</span><span class='Modifier2'>∘</span><span class='Function'>⍉</span></code>, can typically be undone, but one that uses the argument in two different ways, such as <code><span class='Function'>⊢+⋆</span></code>, probably can't.</p>
 <p>A few notable inverses are the <a href="arithmetic.html#basic-arithmetic">logarithm</a> <code><span class='Function'>⋆</span><span class='Modifier'>⁼</span></code>, <a href="transpose.html">un-Transpose</a> <code><span class='Function'>⍉</span><span class='Modifier'>⁼</span></code>, and <a href="replicate.html#inverse">Indices inverse</a> <code><span class='Function'>/</span><span class='Modifier'>⁼</span></code>. <a href="enclose.html">Enclose</a> inverse, <code><span class='Function'>&lt;</span><span class='Modifier'>⁼</span></code>, is an alternative to <a href="pick.html#first">First</a> that requires its argument to be a unit array.</p>
 <p>Structural functions like <a href="take.html">Take</a> and <a href="shift.html">shifts</a> that remove elements from <code><span class='Value'>𝕩</span></code> can't be inverted, because given the result there's no way to know what the elements should be. However, there are two special cases that have inverses defined despite losing data: these are <code><span class='Function'>⊣</span><span class='Modifier'>⁼</span></code> and <code><span class='Value'>k</span><span class='Modifier'>⁼</span></code> where <code><span class='Value'>k</span></code> is a constant (a data type, or <code><span class='Value'>k</span><span class='Modifier'>˙</span></code>). For these, <code><span class='Value'>𝕩</span></code> is required to <a href="match.html">match</a> the always returned value <code><span class='Value'>𝕨</span></code> or <code><span class='Value'>k</span></code>, and this value is also used for the result—even though any result would be valid, as these functions ignore <code><span class='Value'>𝕩</span></code>.</p>
-<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=MyDiiqPigbwgNAozIOKKo+KBvCAz">↗️</a><pre>    <span class='Number'>3</span> <span class='Function'>⊣</span><span class='Modifier'>⁼</span> <span class='Number'>4</span>
+<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=MyDiiqPigbwgNAoKMyDiiqPigbwgMw==">↗️</a><pre>    <span class='Number'>3</span> <span class='Function'>⊣</span><span class='Modifier'>⁼</span> <span class='Number'>4</span>
 <span class='Error'>Error: ⁼: Inverse does not exist</span>
+
     <span class='Number'>3</span> <span class='Function'>⊣</span><span class='Modifier'>⁼</span> <span class='Number'>3</span>
 3
 </pre>
@@ -49,4 +50,8 @@
   <span class='Function'>𝕊</span><span class='Modifier'>⁼</span><span class='Value'>𝕩</span><span class='Head'>:</span> <span class='Value'>𝕩</span><span class='Function'>÷</span><span class='Number'>1</span><span class='Function'>-</span><span class='Value'>𝕩</span>
 <span class='Brace'>}</span>
 </pre>
-<p>The above function could also be defined with the automatically invertible <code><span class='Number'>1</span><span class='Modifier2'>⊸</span><span class='Function'>+</span><span class='Modifier2'>⌾</span><span class='Function'>÷</span></code>, but maybe there's a numerical reason to use the definition above. Like a normal header, an undo header reflects the normal use, but it includes <code><span class='Modifier'>⁼</span></code> and possibly <code><span class='Modifier'>˜</span></code> addition to the function and arguments.</p>
+<p>The above function could also be defined with the automatically invertible <code><span class='Number'>1</span><span class='Modifier2'>⊸</span><span class='Function'>+</span><span class='Modifier2'>⌾</span><span class='Function'>÷</span></code>, but maybe there's a numerical reason to use the definition above. Like a normal header, an undo header reflects the normal use, but it includes <code><span class='Modifier'>⁼</span></code> and possibly <code><span class='Modifier'>˜</span></code> addition to the function and arguments. Any header that includes a function can have these modifiers added—even a fancy modifier header like <code><span class='Function'>𝔽</span><span class='Modifier'>_m1⁼</span><span class='Value'>a</span><span class='Ligature'>‿</span><span class='Value'>b</span><span class='Head'>:</span></code> or plain label like <code><span class='Function'>𝕊</span><span class='Modifier'>⁼</span><span class='Head'>:</span></code>. The three forms are <code><span class='Function'>𝕊</span><span class='Modifier'>⁼</span><span class='Value'>𝕩</span><span class='Head'>:</span></code>, <code><span class='Value'>𝕨</span><span class='Function'>𝕊</span><span class='Modifier'>⁼</span><span class='Value'>𝕩</span><span class='Head'>:</span></code>, and <code><span class='Value'>𝕨</span><span class='Function'>𝕊</span><span class='Modifier'>˜⁼</span><span class='Value'>𝕩</span><span class='Head'>:</span></code>, and the two dyadic forms are also used to derive <code><span class='Value'>k</span><span class='Modifier2'>⊸</span><span class='Function'>F</span><span class='Modifier'>⁼</span><span class='Value'>𝕩</span></code> and <code><span class='Function'>F</span><span class='Modifier2'>⟜</span><span class='Value'>k</span><span class='Modifier'>⁼</span><span class='Value'>𝕩</span></code> with a constant <code><span class='Value'>k</span></code>.</p>
+<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=RyDihpAgeyDwnZWKy5zigbw68J2VqS0yw5fwnZWoIDsg8J2VqCsyw5fwnZWpIH0KR+KfnDLigbwgNw==">↗️</a><pre>    <span class='Function'>G</span> <span class='Gets'>←</span> <span class='Brace'>{</span> <span class='Function'>𝕊</span><span class='Modifier'>˜⁼</span><span class='Head'>:</span><span class='Value'>𝕩</span><span class='Function'>-</span><span class='Number'>2</span><span class='Function'>×</span><span class='Value'>𝕨</span> <span class='Head'>;</span> <span class='Value'>𝕨</span><span class='Function'>+</span><span class='Number'>2</span><span class='Function'>×</span><span class='Value'>𝕩</span> <span class='Brace'>}</span>
+    <span class='Function'>G</span><span class='Modifier2'>⟜</span><span class='Number'>2</span><span class='Modifier'>⁼</span> <span class='Number'>7</span>
+3
+</pre>