Normative: Correctly capture a number negation prefix

Fixes #44

spec.html

@@ -258,14 +258,23 @@ <h1>
         <emu-alg>
           1. Let _F_ be the active function object.
           1. Assert: _F_ is a `JSON.parse` built-in function object (see <emu-xref href="#sec-json.parse">JSON.parse</emu-xref>).
-          1. Let _types_ be &laquo; |NullLiteral|, |BooleanLiteral|, |NumericLiteral|, |StringLiteral|, |ArrayLiteral|, |ObjectLiteral| &raquo;.
+          1. Let _types_ be &laquo; |NullLiteral|, |BooleanLiteral|, |NumericLiteral|, |StringLiteral|, |ArrayLiteral|, |ObjectLiteral|, |UnaryExpression| &raquo;.
+          1. Let _unaryExpression_ be ~empty~.
           1. Let _queue_ be &laquo; this Parse Node &raquo;.
           1. Repeat, while _queue_ is not empty,
             1. Let _candidate_ be the first element of _queue_.
             1. Remove the first element from _queue_.
             1. Let _queuedChildren_ be *false*.
             1. For each nonterminal _type_ of _types_, do
-              1. If _candidate_ is an instance of _type_, return _candidate_.
+              1. If _candidate_ is an instance of _type_, then
+                1. NOTE: In the JSON grammar, a <code>number</code> token may represent a negative value. In ECMAScript, negation is represented as a unary operation.
+                1. If _type_ is |UnaryExpression|, then
+                  1. Set _unaryExpression_ to _candidate_.
+                1. Else if _type_ is |NumericLiteral|, then
+                  1. Assert: _unaryExpression_ Contains _candidate_ is *true*.
+                  1. Return _unaryExpression_.
+                1. Else,
+                  1. Return _candidate_.
               1. If _queuedChildren_ is *false* and _candidate_ is an instance of a nonterminal and _candidate_ Contains _type_ is *true*, then
                 1. Let _children_ be a List containing each child node of _candidate_, in order.
                 1. Set _queue_ to the list-concatenation of _queue_ and _children_.