add prolactin’s biological activity “density” (µg/mIU)

This commit defines international units (IU) of biological activity,
as well as the biological activity “density” of the current reference
preparation of prolactin, to make it easier to convert prolactin
levels between ng/mL (as preferred in the US) and mIU/L (as preferred
in Australia).

I can’t find a formal term for the relationship between biological
activity and mass in reference preparations, so I’ve chosen “density”
by analogy with linear_density, area_density, etc. For example:

> 256 mIU/L * biological_activity_density of prolactin -> ng/mL
4096/335, approx. 12.22686 nanogram / milliliter (density)

definitions.units

@@ -207,6 +207,23 @@ sr        !steradian
 ?? that the random variable takes on the value i.
 bit       !
 
+?? One international unit of a biological agent is the biological
+?? activity equivalent to a standard amount of a substance that
+?? represents the agent (its “reference preparation”). Agents that
+?? can be measured in international units are generally found in
+?? multiple forms (such as Vitamin A) and/or not meaningfully
+?? comparable in SI units like moles (such as prolactin).
+?? WHO TRS 932, Annex 2.
+IU                              !
+
+# XXX: conflates “biological activity” and “immunological activity” to
+# avoid a conflicting quantities warning; some biological agents have
+# more than one reference preparation and/or more than one type of
+# biological activity [WHO TRS 932 pp. 80–81], but it’s unclear how
+# we would map such agents to Rink’s substance/property model
+biological_activity             ? IU
+biological_activity_density     ? mass / biological_activity
+
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 #                                                                         #
 # Prefixes (longer names must come first)                                 #
@@ -7041,6 +7058,14 @@ phenyl {
     molar_mass      mass 77.1057 g / amount mol
 }
 
+# Biological or immunological activity of reference preparations.
+
+prolactin {
+    # 4th International Standard for Prolactin, human, pituitary [WHO/BS/2016.2292]
+    # <https://apps.who.int/iris/handle/10665/253053>
+    biological_activity_density     mass 3.2 µg / biological_activity 67 mIU
+}
+
 !endcategory
 
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