This calculator evaluates expressions written according to the postfix notation
e.g.: 4 4 - cos -41 -
which is the same as: cos(4-4)-(-41)

It's OK! the program contains a helpful keyword: help
it can be used like so:
rpolcalc help
or
rpolcalc help *the operator you need help with*

Great
just type

git clone https://github.com/lorlouis/RpolCalc.git
cd RpolCalc
./configure
make
sudo make install

if you are using an arch-based distro you can simply install this program from the AUR.
yay -S rpolcalc-git