Consider the following Python algorithm:
def calculate_tax(amount, region):
if region == "UK":
return amount * 0.20
return amount * 0.15
subtotal = float(input("Enter subtotal: "))
tax_amount = calculate_tax(subtotal, "UK")
total = subtotal + tax_amount
print("Total is: " + str(total))
Identify the name of a user-defined subprogram used in this algorithm.