<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def youtube(keyword=None):
 """Open youtube. Args: keyword (optional):
 Search word. """

    if keyword is None:
        web.open('https://www.youtube.com/watch?v=L_mBVT2jBFw')
    else:
        web.open(quote('https://www.youtube.com/results?search_query={}'.format(keyword), RESERVED))