d represents the driver. The value of d may be M or D.
p represents the passenger. The value of p may be M, D, or C.

d: Put d on the boat, cross to the other shore with d on board, then put d on the shore. For example M is a command.

dp: Put dp on the boat, cross to the other shore with dp on board, then put dp on the shore. For example MC is a command.

END: End the program and reset the program to enable reexecution of commands with step or run buttons.