master 2c6015437e7 1/2: * lisp/calc/calc-units.el (math-standard-units): Document.

[Mattias Engdegård]

branch: master
commit 2c6015437e7fbb1f052a87ea37133b9240018c0d
Author: Mattias Engdegård <mattiase@acm.org>
Commit: Mattias Engdegård <mattiase@acm.org>

    * 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.
---
 lisp/calc/calc-units.el | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/lisp/calc/calc-units.el b/lisp/calc/calc-units.el
index 988fef2fcd2..c2f692007d5 100644
--- 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.