Add some semantics description

src/haasdaiga/semantics/expressions.md

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+# Fixed Expressions 
+
+| Purpose   | English | Hàäsdáïga literal translation[^1] |
+|-----------|---------|---------------------|
+| Greeting  | Hello   |                     |
+| Apologize | Sorry   | I hope that you me.acc forgive.sub.fut |
+| Express sympathy | Sorry |                     |
+
+[^1]: For the sake of simplicity and generality, only noun cases (if not
+  nominative) and verb mood (if not indicative) is used.

src/haasdaiga/semantics/name.md

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+# Proper name and translation
+
+## Given name gender
+
+As a tradition, ancient Gaidaanan people gave names to newborns based with
+gender based on the month of the year, as did people during the story
+timeline.  This is obvious with Hàäsdáïga month names, but it is not that
+transparent to use in our world, which use 12 months in both solar and lunar
+calendar (I am not aware of any other calendar system).  For that reason, I
+propose that we instead decide based on lunar months:
+
+- Wood if the person is born from month 1-3
+- Fire if the person is born from month 4-6
+- Metal if the person is born from month 7-9
+- Water if the person is born from month 10-12
+
+This corresponds to the four seasons.  Optionally, people born in month 6 or 7
+can be assigned gender Earth.
+
+## Family name gender
+
+Family names usually have meaning.  If it is a noun (e.g. Poisson, Rice, Wood,
+Pierre, Lagrange, ...) or adjective (Legrand, Leblanc, Weiss), they carry the
+same gender as that corresponding noun or verb, even if it is not translated.
+If it is a profession, which does not have a default gender, it would
+theoretically be the gender of the first person carrying that name.  That is
+however not feasible, so instead it is associated with the thing that
+profession works on.  For example, Smith → metal, Gärtner → wood.  For names of
+unknown origin (Johnson, James, Nguyễn), just choose the first vowel in the
+name for gender.
+
+All family names in Hàäsdáïga are nouns, though most people don't use it.

src/haasdaiga/semantics/time.md

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+# Time
+
+## Months
+
+In the Gaïdaäna calendar, there are ten months, named after the five phases
+and yin-yang:
+
+- Yang Wood
+- Yin Wood
+- Yang Fire
+- Yin Fire
+- Yang Earth
+- Yin Earth
+- Yang Metal
+- Yin Metal
+- Yang Water
+- Yin Water
+
+Colloquially, they are also called by number from 1 to 10.