Cleanup echo/isset/unset grammar

isset and unset accept only variables.
For echo, merge the single and multiple expression cases.

Author: nikic

spec/10-expressions.md

@@ -333,12 +333,11 @@ other values or expressions could be used.
 
 <pre>
   <i>echo-intrinsic:</i>
-    echo  <i>expression</i>
-    echo  <i>expression-list-two-or-more</i>
+    echo  <i>expression-list</i>
 
   <i>expression-list-two-or-more:</i>
-    <i>expression</i>  ,  <i>expression</i>
-    <i>expression-list-two-or-more</i>  ,  <i>expression</i>
+    <i>expression</i>
+    <i>expression-list</i>  ,  <i>expression</i>
 </pre>
 
 **Defined elsewhere**
@@ -508,25 +507,21 @@ exit;
 
 <pre>
   <i>isset-intrinsic:</i>
-    isset  (  <i>expression-list-one-or-more</i>  )
+    isset  (  <i>variable-list</i>  )
 
-  <i>expression-list-one-or-more</i>:
-    <i>expression</i>
-    <i>expression-list-one-or-more</i>  ,  <i>expression</i>
+  <i>variable-list</i>:
+    <i>variable</i>
+    <i>variable-list</i>  ,  <i>variable</i>
 </pre>
 
 **Defined elsewhere**
 
-* [*expression*](#general-6)
-
-**Constraints**
-
-Each *expression* must designate a variable.
+* [*variable*](#variables)
 
 **Semantics**
 
 This intrinsic returns `TRUE` if all the variables designated by
-*expression*s are set and their values are not `NULL`. Otherwise, it
+*variables*s are set and their values are not `NULL`. Otherwise, it
 returns `FALSE`.
 
 If this intrinsic is used with an expression that designate a [dynamic
@@ -708,21 +703,17 @@ $a > $b ? print "..." : print "...";
 
 <pre>
   <i>unset-intrinsic:</i>
-    unset  (  <i>expression-list-one-or-more</i>  )
+    unset  (  <i>variable-list</i>  )
 </pre>
 
 **Defined elsewhere**
 
-* [*expression-list-one-or-more*](#isset)
-
-**Constraints**
-
-Each *expression* must designate a variable.
+* [*variable-list*](#isset)
 
 **Semantics**
 
 This intrinsic [unsets](07-variables.md#general) the variables designated by each
-*expression* in *expression-list-one-or-more*. No value is returned. An
+*variable* in *variable-list*. No value is returned. An
 attempt to unset a non-existent variable (such as a non-existent element
 in an array) is ignored.
 

spec/19-grammar.md

@@ -393,12 +393,11 @@ The grammar notation is described in [Grammars section](09-lexical-structure.md#
     <i>print-intrinsic</i>
 
   <i>echo-intrinsic:</i>
-    echo  <i>expression</i>
-    echo  <i>expression-list-two-or-more</i>
+    echo  <i>expression-list</i>
 
   <i>expression-list-two-or-more:</i>
-    <i>expression</i>  ,  <i>expression</i>
-    <i>expression-list-two-or-more</i>  ,  <i>expression</i>
+    <i>expression</i>
+    <i>expression-list</i>  ,  <i>expression</i>
 
   <i>empty-intrinsic:</i>
     empty ( <i>expression</i>  )
@@ -413,11 +412,11 @@ The grammar notation is described in [Grammars section](09-lexical-structure.md#
     die   (   <i>expression<sub>opt</sub></i>   )
 
   <i>isset-intrinsic:</i>
-    isset  (  <i>expression-list-one-or-more</i>  )
+    isset  (  <i>variable-list</i>  )
 
-  <i>expression-list-one-or-more</i>:
-    <i>expression</i>
-    <i>expression-list-one-or-more</i>  ,  <i>expression</i>
+  <i>variable-list</i>:
+    <i>variable</i>
+    <i>variable-list</i>  ,  <i>variable</i>
 
   <i>list-intrinsic:</i>
     list  (  <i>list-expression-list<sub>opt</sub></i>  )
@@ -443,7 +442,7 @@ The grammar notation is described in [Grammars section](09-lexical-structure.md#
     print  <i>expression</i>
 
   <i>unset-intrinsic:</i>
-    unset  (  <i>expression-list-one-or-more</i>  )
+    unset  (  <i>variable-list</i>  )
 
   <i>anonymous-function-creation-expression:</i>
   static<sub>opt</sub> function  &<sub>opt</sub> (  <i>parameter-declaration-list<sub>opt<sub></i>  ) <i>return-type<sub>opt</sub></i> <i>anonymous-function-use-clause<sub>opt</sub></i>