val defn with ids

Problem
-------
`val` definitions with multiple identifiers are not supported.

Solution
--------
This implements `identifiers`, which acts like a pattern.

corpus/definitions.txt

@@ -852,6 +852,7 @@ Value definitions
 class A {
   val b = 1
   val c : String = "d"
+  val a1, a2 = pair
 }
 
 --------------------------------------------------------------------------------
@@ -866,7 +867,12 @@ class A {
       (val_definition
         (identifier)
         (type_identifier)
-        (string)))))
+        (string))
+      (val_definition
+        (identifiers
+          (identifier)
+          (identifier))
+        (identifier)))))
 
 ================================================================================
 Variable declarations

grammar.js

@@ -417,7 +417,10 @@ module.exports = grammar({
 
     val_definition: $ => prec(PREC.binding_def, seq(
       $._start_val,
-      field('pattern', $._pattern),
+      field('pattern', choice(
+        $._pattern,
+        $.identifiers
+      )),
       optional(seq(':', field('type', $._type))),
       '=',
       field('value', $._indentable_expression)
@@ -1293,6 +1296,12 @@ module.exports = grammar({
 
     _identifier: $ => choice($.identifier, $.operator_identifier),
 
+    identifiers: $ => prec.left(-1, seq(
+      $.identifier,
+      ',',
+      commaSep1($.identifier)
+    )),
+
     wildcard: $ => '_',
 
     /**