From 701226ca02b0ef5c8b1cdd85f06d21fade3356be Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?I=C3=B1aki=20=C3=9Acar?= <iucar@fedoraproject.org>
Date: Sun, 8 Nov 2020 00:49:10 +0100
Subject: [PATCH] add some notes about #253

---
 R/arith.R        | 12 +++++++++++-
 man/Ops.units.Rd | 13 ++++++++++++-
 2 files changed, 23 insertions(+), 2 deletions(-)

diff --git a/R/arith.R b/R/arith.R
index d3614998..a412e0e6 100644
--- a/R/arith.R
+++ b/R/arith.R
@@ -5,7 +5,17 @@
 
 #' S3 Ops Group Generic Functions for units objects
 #'
-#' Ops functions for units objects, including comparison, product and divide, add, subtract
+#' Ops functions for units objects, including comparison, product and divide,
+#' add, subtract.
+#'
+#' Users are advised against performing arithmetical operations with
+#' temperatures in different units. The \pkg{units} package ensure that results
+#' 1) are arithmetically correct, and 2) satisfy dimensional analysis, but could
+#' never ensure that results are physically meaningful. Temperature units are
+#' special because there is an absolute unit, Kelvin, and relative ones, Celsius
+#' and Fahrenheit degrees. Arithmetic operations between them are meaningless
+#' from the physical standpoint. Users are thus advised to convert all
+#' temperatures to Kelvin before operating.
 #'
 #' @param e1 object of class \code{units}, 
 #'        or something that can be coerced to it by \code{as_units(e1)}
diff --git a/man/Ops.units.Rd b/man/Ops.units.Rd
index 593b1fd4..31874571 100644
--- a/man/Ops.units.Rd
+++ b/man/Ops.units.Rd
@@ -18,7 +18,18 @@ or in case of power a number (integer n or 1/n)}
 object of class \code{units}
 }
 \description{
-Ops functions for units objects, including comparison, product and divide, add, subtract
+Ops functions for units objects, including comparison, product and divide,
+add, subtract.
+}
+\details{
+Users are advised against performing arithmetical operations with
+temperatures in different units. The \pkg{units} package ensure that results
+1) are arithmetically correct, and 2) satisfy dimensional analysis, but could
+never ensure that results are physically meaningful. Temperature units are
+special because there is an absolute unit, Kelvin, and relative ones, Celsius
+and Fahrenheit degrees. Arithmetic operations between them are meaningless
+from the physical standpoint. Users are thus advised to convert all
+temperatures to Kelvin before operating.
 }
 \examples{
 a <- set_units(1:3, m/s)