Editorial: add a paragraph defining Parse Node etc.

spec.html

@@ -439,6 +439,7 @@ <h1>The Numeric String Grammar</h1>
       <h1>The Syntactic Grammar</h1>
       <p>The <em>syntactic grammar</em> for ECMAScript is given in clauses 11, 12, 13, 14, and 15. This grammar has ECMAScript tokens defined by the lexical grammar as its terminal symbols (<emu-xref href="#sec-lexical-and-regexp-grammars"></emu-xref>). It defines a set of productions, starting from two alternative goal symbols |Script| and |Module|, that describe how sequences of tokens form syntactically correct independent components of ECMAScript programs.</p>
       <p>When a stream of code points is to be parsed as an ECMAScript |Script| or |Module|, it is first converted to a stream of input elements by repeated application of the lexical grammar; this stream of input elements is then parsed by a single application of the syntactic grammar. The input stream is syntactically in error if the tokens in the stream of input elements cannot be parsed as a single instance of the goal nonterminal (|Script| or |Module|), with no tokens left over.</p>
+      <p>When a parse is successful, it constructs a <em>parse tree</em>, a rooted tree structure in which each node is a <dfn>Parse Node</dfn>. Each Parse Node is an <em>instance</em> of a symbol in the grammar; it represents a span of the source text that can be derived from that symbol. The root node of the parse tree, representing the whole of the source text, is an instance of the parse's goal symbol. When a Parse Node is an instance of a nonterminal, it is also an instance of some production that has that nonterminal as its left-hand side. Moreover, it has zero or more <em>children</em>, one for each symbol on the production's right-hand side: each child is a Parse Node that is an instance of the corresponding symbol.</p>
       <p>Productions of the syntactic grammar are distinguished by having just one colon &ldquo;<b>:</b>&rdquo; as punctuation.</p>
       <p>The syntactic grammar as presented in clauses 12, 13, 14 and 15 is not a complete account of which token sequences are accepted as a correct ECMAScript |Script| or |Module|. Certain additional token sequences are also accepted, namely, those that would be described by the grammar if only semicolons were added to the sequence in certain places (such as before line terminator characters). Furthermore, certain token sequences that are described by the grammar are not considered acceptable if a line terminator character appears in certain &ldquo;awkward&rdquo; places.</p>
       <p>In certain cases in order to avoid ambiguities the syntactic grammar uses generalized productions that permit token sequences that do not form a valid ECMAScript |Script| or |Module|. For example, this technique is used for object literals and object destructuring patterns. In such cases a more restrictive <em>supplemental grammar</em> is provided that further restricts the acceptable token sequences. In certain contexts, when explicitly specified, the input elements corresponding to such a production are parsed again using a goal symbol of a supplemental grammar. The input stream is syntactically in error if the tokens in the stream of input elements parsed by a cover grammar cannot be parsed as a single instance of the corresponding supplemental goal symbol, with no tokens left over.</p>