two programs discussed here: https://camnugent.wordpress.com/2018/03/29/constrained-programming-a-python-program-with-no-equals/

constrained_program.py

# ORIGINAL version


def merge_sorted(list_a, list_b):
	""" take two lists of integers sorted smallest to largest, 
		merge them to a single output list with
		no duplicate values """
	
	output_list = []

	while (len(list_a) > 0) and (len(list_b) > 0):
		if list_a[0] == list_b[0]:
			output_list.append(list_a.pop(0))
			list_b.pop(0)
		
		elif list_a[0] < list_b[0]:
			output_list.append(list_a.pop(0))
		
		elif list_a[0] > list_b[0]:	
			output_list.append(list_b.pop(0))

	if len(list_a) > 0:
		output_list.extend(list_a)
	elif len(list_b) > 0:
		output_list.extend(list_b)
	
	return output_list


# CONSTRAINED version
def constrained_merge_sorted(list_a, list_b): 

    globals().__setitem__('output_list' ,  list_a * 0)
    
    while (len(list_a) is not 0) and (len(list_b) is not 0):
        if (list_a[0]) is (list_b[0]): 
            globals().__setitem__('output_list', output_list + [list_a.pop(0)]) 
            list_b.pop(0)
        elif abs(list_a[0] - list_b[0]) is not (list_a[0] - list_b[0]): 
            globals().__setitem__('output_list', output_list + [list_a.pop(0)])
        elif abs(list_a[0] - list_b[0]) is (list_a[0] - list_b[0]): 
            globals().__setitem__('output_list', output_list + [list_b.pop(0)])
    if len(list_a) is not 0: 
        return output_list + list_a 
    elif len(list_b) is not 0: 
        return output_list + list_b 
    else: 
        return output_list
        


list_a = [1,3,4,5]	
list_b = [2,5,6,7,8]

constrained_merge_sorted(list_a, list_b)

x = [1,2,3,7]
y = [2,3,6,8,9]
merge_sorted(x, y)