<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def pretty_dict_string(d, indent=0):
 """Pretty output of nested dictionaries. """

    s = ''
    for key, value in sorted(d.items()):
        s += '    ' * indent + str(key)
        if isinstance(value, dict):
             s += '\n' + pretty_dict_string(value, indent+1)
        else:
             s += '=' + str(value) + '\n'
    return s