''' Purpose: repeatedly prompt the user until they reply with a yes
        or no. The case of the letters do not matter.
'''

looking_for_a_yes_or_no = True

while ( looking_for_a_yes_or_no ) :

   reply = input( 'Enter (yes / no): ' )

   reply = reply.lower()

   if ( reply in [ 'yes', 'no' ] ) :

       looking_for_a_yes_or_no = False

print( reply )