Write a function, print_histogram(a_dict), which takes a Python dictionary as a parameter. The keys of the dictionary are single letters and the corresponding values are integers, e.g., {'b': 5, 'a': 6, 'c': 3}. The function should print a histogram shown as below. Note: the keys are printed in alphabetical order and the number of stars printed is given by the value corresponding to the key. If the parameter dictionary is empty, the function prints nothing.

For example:

print_histogram({'c':3, 'a':6, 'b':5})

Results:

a : ****** b : ***** c : ***