def subtract(x, y):
    return x - y

def add(x, y):
    return x + y

def multiply(x, y):
    return x*y

def exponent(x, y):
    return x**y

def apply_function(function, x, y):
    return function(x, y)

def apply_function_to_list(f, some_list):
    result = []
    
    for val in some_list:
        result.append( f(val) )
        
    return result

def double(x):
    return 2 * x

def filter_list(some_function, some_list):
    result = []
    
    for val in some_list:
        if some_function(val):
            result.append(val)
    
    return result

def is_even(num):
    return num % 2 == 0