EMResponder.h
As mentioned earlier, the EMResponder will be responsible for doing the brunt of the calculations. The header sets up several enumerated types (to make the code a little easier to read in places) and the prototypes for the class methods.

EMResponder.h 

#import <Foundation/Foundation.h>

typedef enum Op_Type 
{
    NO_OP 	= 0,		// no operator
    ADD_OP 	= 1,		// addition
    SUBTRACT_OP = 2,		// subtraction
    MULTIPLY_OP = 3, 		// multiplication
    DIVIDE_OP 	= 4,    	// division
    EXPONENT_OP = 5,		// exponent
    XROOT_OP 	= 6,		// taking the root to the xth degree
    MOD_OP 	= 7,		// modulus division
    EE_OP 	= 8,		// EE operator
    NPR_OP 	= 9,		// permuations
    NCR_OP 	= 10		// combinations
} OpType;

typedef enum Angle_Type
{
    DEGREE 	 = 0,
    RADIAN 	 = 1,
    GRADIENT     = 2
} AngleType;

@interface EMResponder : NSObject 
{
    double current_value;       // the current number (which is being edited)
    double previous_value;      // the other operand (previous operand)
    double e_value;             // the number e
    OpType op_type;             // the current operator
    AngleType angle_type;       // type of angle used (radian, degree, gradient)
    int trailing_digits;        // used in decimal number input
    BOOL startNewDigit;         // allow new number in display
}

@end

// class method prototypes