Clean up lhs grammar spec

2 files changed, 15 insertions, 10 deletions

diff --git a/docs/spec/grammar.html b/docs/spec/grammar.html
index 9f99b430..3b18a102 100644
--- a/docs/spec/grammar.html
+++ b/docs/spec/grammar.html
@@ -44,12 +44,17 @@
 </span><span class='Function'>FuncExpr</span> <span class='Function'>=</span> <span class='Function'>Train</span>
          <span class='Function'>|</span> <span class='Function'>F</span> <span class='Function'>ASGN</span> <span class='Function'>FuncExpr</span>
 </pre>
-<p>Subject expressions are complicated by the possibility of list and namespace assignment, which share the nodes <code><span class='Value'>lhsList</span></code> and <code><span class='Value'>lhsStr</span></code> and cannot be completely distinguished until execution. We also define nothing-statements, which have very similar syntax to subject expressions but do not permit assignment.</p>
+<p>Subject expressions consist mainly of function application. We also define nothing-statements, which have very similar syntax to subject expressions but do not permit assignment. They can be used as an <code><span class='Function'>STMT</span></code> or in place of a left argument.</p>
 <pre><span class='Value'>arg</span>      <span class='Function'>=</span> <span class='Value'>subExpr</span>
          <span class='Function'>|</span> <span class='Paren'>(</span> <span class='Value'>subject</span> <span class='Function'>|</span> <span class='Value'>nothing</span> <span class='Paren'>)</span><span class='Value'>?</span> <span class='Function'>Derv</span> <span class='Value'>arg</span>
 <span class='Value'>nothing</span>  <span class='Function'>=</span> <span class='String'>&quot;·&quot;</span>
          <span class='Function'>|</span> <span class='Paren'>(</span> <span class='Value'>subject</span> <span class='Function'>|</span> <span class='Value'>nothing</span> <span class='Paren'>)</span><span class='Value'>?</span> <span class='Function'>Derv</span> <span class='Value'>nothing</span>
-<span class='Function'>NAME</span>     <span class='Function'>=</span> <span class='Value'>s</span> <span class='Function'>|</span> <span class='Function'>F</span> <span class='Function'>|</span> <span class='Modifier'>_m</span> <span class='Function'>|</span> <span class='Modifier2'>_c_</span>
+<span class='Value'>subExpr</span>  <span class='Function'>=</span> <span class='Value'>arg</span>
+         <span class='Function'>|</span> <span class='Value'>lhs</span> <span class='Function'>ASGN</span> <span class='Value'>subExpr</span>
+         <span class='Function'>|</span> <span class='Value'>lhs</span> <span class='Function'>Derv</span> <span class='String'>&quot;↩&quot;</span> <span class='Value'>subExpr?</span>      <span class='Comment'># Modified assignment
+</span></pre>
+<p>The target of subject assignment can be compound to allow for destructuring. List and namespace assignment share the nodes <code><span class='Value'>lhsList</span></code> and <code><span class='Value'>lhsStr</span></code> and cannot be completely distinguished until execution. The term <code><span class='Value'>sl</span></code> in <code><span class='Function'>LHS_SUB</span></code> is used for header inputs below: as an additional rule, it cannot be used in the <code><span class='Value'>lhs</span></code> term of a <code><span class='Value'>subExpr</span></code> node.</p>
+<pre><span class='Function'>NAME</span>     <span class='Function'>=</span> <span class='Value'>s</span> <span class='Function'>|</span> <span class='Function'>F</span> <span class='Function'>|</span> <span class='Modifier'>_m</span> <span class='Function'>|</span> <span class='Modifier2'>_c_</span>
 <span class='Function'>LHS_SUB</span>  <span class='Function'>=</span> <span class='String'>&quot;·&quot;</span> <span class='Function'>|</span> <span class='Value'>lhsList</span> <span class='Function'>|</span> <span class='Value'>sl</span>
 <span class='Function'>LHS_ANY</span>  <span class='Function'>=</span> <span class='Function'>NAME</span> <span class='Function'>|</span> <span class='Function'>LHS_SUB</span> <span class='Function'>|</span> <span class='String'>&quot;(&quot;</span> <span class='Function'>LHS_ELT</span> <span class='String'>&quot;)&quot;</span>
 <span class='Function'>LHS_ATOM</span> <span class='Function'>=</span> <span class='Function'>LHS_ANY</span> <span class='Function'>|</span> <span class='String'>&quot;(&quot;</span> <span class='Value'>lhsStr</span> <span class='String'>&quot;)&quot;</span>
@@ -59,10 +64,7 @@
 <span class='Value'>lhsList</span>  <span class='Function'>=</span> <span class='String'>&quot;⟨&quot;</span> <span class='Separator'>⋄</span><span class='Value'>?</span> <span class='Paren'>(</span> <span class='Paren'>(</span> <span class='Function'>LHS_ENTRY</span> <span class='Separator'>⋄</span> <span class='Paren'>)</span><span class='Value'>*</span> <span class='Function'>LHS_ENTRY</span> <span class='Separator'>⋄</span><span class='Value'>?</span> <span class='Paren'>)</span><span class='Value'>?</span> <span class='String'>&quot;⟩&quot;</span>
 <span class='Value'>lhsComp</span>  <span class='Function'>=</span> <span class='Function'>LHS_SUB</span> <span class='Function'>|</span> <span class='Value'>lhsStr</span> <span class='Function'>|</span> <span class='String'>&quot;(&quot;</span> <span class='Value'>lhs</span> <span class='String'>&quot;)&quot;</span>
 <span class='Value'>lhs</span>      <span class='Function'>=</span> <span class='Value'>s</span> <span class='Function'>|</span> <span class='Value'>lhsComp</span>
-<span class='Value'>subExpr</span>  <span class='Function'>=</span> <span class='Value'>arg</span>
-         <span class='Function'>|</span> <span class='Value'>lhs</span> <span class='Function'>ASGN</span> <span class='Value'>subExpr</span>
-         <span class='Function'>|</span> <span class='Value'>lhs</span> <span class='Function'>Derv</span> <span class='String'>&quot;↩&quot;</span> <span class='Value'>subExpr?</span>      <span class='Comment'># Modified assignment
-</span></pre>
+</pre>
 <p>A header looks like a name for the thing being headed, or its application to inputs (possibly twice in the case of modifiers). As with assignment, it is restricted to a simple form with no extra parentheses. The full list syntax is allowed for arguments. A plain name is called a label and can be used for a block with or without arguments. First we define headers <code><span class='Function'>IMM_HEAD</span></code> that include no arguments.</p>
 <pre><span class='Value'>headW</span>    <span class='Function'>=</span> <span class='Value'>lhs</span> <span class='Function'>|</span> <span class='String'>&quot;𝕨&quot;</span>
 <span class='Value'>headX</span>    <span class='Function'>=</span> <span class='Value'>lhs</span> <span class='Function'>|</span> <span class='String'>&quot;𝕩&quot;</span>
diff --git a/spec/grammar.md b/spec/grammar.md
index 90f4718b..df355c77 100644
--- a/spec/grammar.md
+++ b/spec/grammar.md
@@ -47,12 +47,18 @@ Functions can be formed by applying modifiers, or with trains. Modifiers are lef
     FuncExpr = Train
              | F ASGN FuncExpr
 
-Subject expressions are complicated by the possibility of list and namespace assignment, which share the nodes `lhsList` and `lhsStr` and cannot be completely distinguished until execution. We also define nothing-statements, which have very similar syntax to subject expressions but do not permit assignment.
+Subject expressions consist mainly of function application. We also define nothing-statements, which have very similar syntax to subject expressions but do not permit assignment. They can be used as an `STMT` or in place of a left argument.
 
     arg      = subExpr
              | ( subject | nothing )? Derv arg
     nothing  = "·"
              | ( subject | nothing )? Derv nothing
+    subExpr  = arg
+             | lhs ASGN subExpr
+             | lhs Derv "↩" subExpr?      # Modified assignment
+
+The target of subject assignment can be compound to allow for destructuring. List and namespace assignment share the nodes `lhsList` and `lhsStr` and cannot be completely distinguished until execution. The term `sl` in `LHS_SUB` is used for header inputs below: as an additional rule, it cannot be used in the `lhs` term of a `subExpr` node.
+
     NAME     = s | F | _m | _c_
     LHS_SUB  = "·" | lhsList | sl
     LHS_ANY  = NAME | LHS_SUB | "(" LHS_ELT ")"
@@ -63,9 +69,6 @@ Subject expressions are complicated by the possibility of list and namespace ass
     lhsList  = "⟨" ⋄? ( ( LHS_ENTRY ⋄ )* LHS_ENTRY ⋄? )? "⟩"
     lhsComp  = LHS_SUB | lhsStr | "(" lhs ")"
     lhs      = s | lhsComp
-    subExpr  = arg
-             | lhs ASGN subExpr
-             | lhs Derv "↩" subExpr?      # Modified assignment
 
 A header looks like a name for the thing being headed, or its application to inputs (possibly twice in the case of modifiers). As with assignment, it is restricted to a simple form with no extra parentheses. The full list syntax is allowed for arguments. A plain name is called a label and can be used for a block with or without arguments. First we define headers `IMM_HEAD` that include no arguments.