Start draft of KDL Data Model

MODEL.md

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+# KDL Data Model
+
+This document specifies an abstract data model of KDL document language. 
+
+*This is version `0.0.0` of KDL Data model. It has not been released yet.*
+
+## Introduction
+
+KDL is defined by [KDL Specification](SPEC.md) as a formal language with
+components such as nodes, identifiers, strings, comments, and whitespace.
+Some of these components can be expressed in mutliple ways and some of
+these components are typically ignored when a KDL document is parsed.
+
+KDL Data model defines a conceptual structure of semantically relevant elements
+expressed in KDL syntax. The data model is required to process KDL documents
+independent from syntax and implementations with [KDL Query
+Language](QUERY-SPEC.md) and [KDL Schema Language](SCHEMA-SPEC.md).
+
+## Elements
+
+Every KDL document represents a [document](#document).
+
+Sequences and sets can be empty.
+
+A Unicode string is a sequence of Unicode code points.
+
+### Document
+
+A document is a sequence of [nodes](#node).
+
+Documents must not contain itself via node direct or indirect children to avoid
+circular structures.
+
+### Node
+
+A node consists of three mandatory and two optional elements:
+
+* a **name** being a Unicode string
+* an optional **tag** being a Unicode string
+* a sequence of **arguments**, each being a [value](#value)
+* a set of **properties**, each consisting of
+    * a name being Unicode string unique within the set
+    * a [value](#value)
+* an optional list of **children** being a [document](#document)
+
+### Value
+
+A value is one of:
+
+* a Unicode string
+* a **number**, consisting of an integer part and a decimal part
+* a **boolean**, being one of the special values *true* and *false*
+* the special value *null*
+
+## Application Notes
+
+Implementations may want to limit the set of processable KDL documents by
+limiting properties of the data model such as the following:
+
+* A node with empty string tag (e.g. `("")node`) differs from a node without
+  tag (`node`).
+
+* A node with empty children (e.g. `node {}`) differs from a node without
+  children (`node`).
+
+* KDL does not differ between integer numbers and numbers with fractional part
+  nor does it define a fixed limit to the length or precision of numbers.
+
+* property names differ also if their distinct strings become equivalent after
+  Unicode normalization.
+
+Applications must make clear whether they support full KDL data model or a
+specific subset and whether they modify KDL documents to fit to their limited
+model. Applications may further extend their document model with additional
+information such as line numbers beyond the scope of this specification.
+