generate_numbered_unique_names() effectively ignores the counter argument

It always starts counting at 0:

def generate_numbered_unique_names(prefix, num=None):                                                                                                         
    orig_num = num                                                                                                                                            
    num = 0                                                                                                                                                   
    if orig_num is None:                                                                                                                                      
        yield (num, prefix)                                                                                                                                   
                                                                                                                                                              
    while True:                                                                                                                                               
        name = "%s_%d" % (prefix, num)                                                                                                                        
        num += 1                                                                                                                                              
        yield (num, name)                                                                                                                                     

This is in part a cause of loopy#110 (closed).