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diff --git a/tcllib/modules/pt/include/concept.inc b/tcllib/modules/pt/include/concept.inc
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+[comment {
+	 Description of the concepts used in parsing expression
+	 grammars and how their relate to each other. This is useful
+	 to understand the chosen serialization.
+}]
+[list_begin enumerated]
+[enum]
+
+A [term {parsing expression grammar}] consists of a
+[term {start parsing expression}] and a (possibly empty) list of
+[term rules].
+
+[enum]
+Each rule defines a nonterminal symbol of the grammar, with its name,
+semantic mode, and sentennial structure. The latter is provided by a
+[term {parsing expression}].
+
+[enum]
+Each nonterminal symbol is specified at most once, with its name as
+the identifying part.
+
+[list_end]