A drink dispenser requires the user to insert a coin ($\bar c$), then press one of three buttons: $\bar d_{\text{tea}}$ requests a cup of tea $e_{\text{tea}}$, ditto for coffee, and $\bar r$ requests a refund (i.e. the machine gives back the coin: $\bar b$). This dispenser can be modeled by the following CCS process:
$$ M \stackrel{\mathrm{def}}= c.(d_{\text{tea}}.\bar e_{\text{tea}}.M + d_{\text{coffee}}.\bar e_{\text{coffee}}.M + r.\bar b.M)$$
A civil war raises the price of coffee to two coins, while the price of tea remains one coin. We want a modified machine that delivers coffee only after two coins, and acquiesces to a refund after either one or two coins. How can we model the modified machine with a CCS process?