I thought a bit longer about it, and I actually think it's not really a big
bug. It think the way the units system is meant to be used is like this:
sage: var("H_l, h_c,T_a,T_l")
(H_l, h_c, T_a, T_l)
sage:
sol=solve([H_l*units.energy.calorie/units.length.centimeter^2/units.time.minute
== h_c*((T_a - T_l)*units.temperature.kelvin)],h_c)
sage: sol
[h_c == H_l*calorie/(T_a*centimeter^2*kelvin*minute -
T_l*centimeter^2*kelvin*minute)]
The interpretation of this should be that 2*units.length.meter should be
read as 2 * (1 meter) etc. This interpretation works very well if you have
to convert explicit values. Note that you indeed cannot divide by 1 kelvin -
1 kelvin = 0 kelvin. The framework will give sensible answers if you fill in
distinct values for T_a and T_l.
sage: sol[0](T_l=1,T_a=2,H_l=3)
h_c == 3*calorie/(centimeter^2*kelvin*minute)
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