* lisp/calc/calc-units.el (math-standard-units): Document.

Some guesswork, but at least it helps someone trying to make sense of
it next time.

diff --git a/lisp/calc/calc-units.el b/lisp/calc/calc-units.el
index 988fef2fcd20218ff28add43b6db732112f3a7d3..c2f692007d5d702f662cdca2bbf388e7e02061e8 100644 (file)
--- a/lisp/calc/calc-units.el
+++ b/lisp/calc/calc-units.el
@@ -307,8 +307,22 @@
               "22.710947 10^-3 m^3/mol (*)")
     ;; Logarithmic units
     ( Np      nil    "*Neper")
-    ( dB      "(ln(10)/20) Np" "decibel")))
-
+    ( dB      "(ln(10)/20) Np" "decibel"))
+  "List of predefined units for Calc.
+
+Each element is (NAME DEF DESC TEMP-UNIT HUMAN-DEF), where:
+
+NAME      is the unit symbol.
+DEF       is a string defining the unit as a Calc expression; nil if base unit.
+DESC      is a string describing the unit (to a human reader).
+          A leading asterisk indicates that the unit is first in its group.
+TEMP-UNIT is an additional symbol tabulated for temperature units (?) or nil.
+HUMAN-DEF is a string defining the unit (to a human reader).
+          If absent or nil, DEF is used.
+
+(*) in HUMAN-DEF means that the definition is approximate, otherwise exact.
+(**) in DESC means that the unit name is different in TeX and LaTeX
+     display modes.")
 
 (defvar math-additional-units nil
   "Additional units table for user-defined units.