The user guide for using "arithmetic.pro"

1. Make sure that Prolog is installed on the target machine.
2. Make sure that "gv.pro" and "arithmetic.pro" are in a same folder.
3. Open prolog then consult "arithmetic.pro".
4. You are ready to start! The program has following functions:

	"declare(Variable, Value).": inserts a variable and binds it with a value.
	"undeclare(Variable)." : unbinds the value of the given variable.
	"value_of(Variable, Value).": returns true if the given variable with this value exists.
	"bind(Variable, NewValue).": binds a new value to the given variable, old value will be discard.
	"display_bindings.": shows all variables and their bound value.
	
	"inc(Variable).": increments the value of the given variable by 1.
	"dec(Variable).": decrements the value of the given variable by 1.
	"add(Variable, Number).": adds Number to the value of given variable.
	"sub(Variable, Number).": subtracts Number from the value of given variable.

	"add(V1,V2,V3).": Performs V1 + V2 = X then declare(V3,X).
	"sub(V1,V2,V3).": Performs V1 - V2 = X then declare(V3,X).
	"mul(V1,V2,V3).": Performs V1 * V2 = X then declare(V3,X).
	"div(V1,V2,V3).": Performs V1 / V2 = X then declare(V3,X).
	"exp(V1,V2,V3).": Performs V1 ^ V2 = X then declare(V3,X).
	
5. Before starting using the program, you need to declare at least two variables.
5. When you finish the program, type "halt." to exit.