emacs calc and unit (or variable) term gathering

[Eric S Fraga <e.fraga@ucl.ac.uk>]

Hello all,

I am using embedded calc to do some simple algebraic calculations in an
org buffer.  I get the following annoying result:

 {{{calc(n1 := 1.500 kmol / min)}}} 
 {{{calc(n2 := 6.800 kmol / min)}}}
 {{{calc(n3 := n1 + n2 => 1.500 kmol / min + 6.800 kmol / min)}}} 

in that the terms in the final expression are not gathered together.  I
would have expected (and other times do get) 8.300 kmol /min as the
result.  Does anybody have any pointers to how to get embedded calc to
gather terms?

If the units were just kmol, say, calc works as I would expect.

thanks,
-- 
Eric S Fraga via Emacs 27.0.50 & org 9.1.14 on Debian buster/sid