RS 3001: string value escapes, empty-string presence, null literal

Core syntax gains a String values section: both quote kinds accept the
escape sequences backslash-quote and backslash-backslash, strictly --
no other escape is defined. The empty string is defined as a present,
zero-length value, distinct from absence. Null joins the primitive
values, and the Instance section records the assignment semantics:
an unassigned optional attribute is absent, an explicit null
assignment denotes the null value.

Author: ronaldtse

sources/lutaml-lang.adoc

@@ -551,8 +551,39 @@ LutaML supports the following primitive values:
 * `Time` (e.g., `2024-01-01T12:00:00+00:00`),
 * `Decimal` (e.g., `3.14159`),
 * `Hash` (e.g., `{ "key": "value" }`)
+* `Null` (e.g., `null`)
 
 
+=== String values
+
+A string value is delimited by double quotes or by single quotes.
+
+The escape sequences `\"` and `\\` are recognized inside both quote
+kinds: `\"` denotes the quote character that delimits the string, and
+`\\` denotes a backslash. No other escape sequence is defined; a
+backslash followed by any other character does not form a string value.
+
+An empty string is distinct from an absent value: `""` denotes a
+present string of length zero.
+
+[example]
+====
+[source,lml]
+----
+class Glaze {
+  attribute name, String
+  attribute description, String
+  attribute note, String
+}
+
+instance "Glaze" {
+  name = "Raku \"kiln\" fired"          // embedded quotes
+  description = 'say "hi" and \'bye\''  // the other quote kind
+  note = ""                                // present, empty
+}
+----
+====
+
 === Declaration, arguments and blocks
 
 A declaration is a keyword that represents an action or a statement.
@@ -1882,6 +1913,10 @@ An `instance` construct is used to define an instance of a class.
 To assign values to attributes, the attribute name is followed by an equal sign
 and the value.
 
+An optional attribute without an assignment is absent. An explicit
+`null` assignment denotes the null value, and an empty string `""`
+denotes a present, empty string.
+
 Syntax:
 
 [source,lml]