def _attach(self, start_component):
r'''Attaches context mark to `start_component`.
Makes sure no context mark of same type is already attached
to score component that starts with start component.
Returns context mark.
'''
from abjad.tools import contexttools
classes = (type(self), )
effective_context_mark = \
start_component._get_effective_context_mark(classes)
if effective_context_mark is not None:
if effective_context_mark.start_component._get_timespan().start_offset == \
start_component._get_timespan().start_offset:
message = 'effective context mark already attached'
message += ' to component starting at same time.'
raise ExtraMarkError(message)
return Mark._attach(self, start_component)
