def single(iterable, pred=None, default=_undefined):
iterator = iter(where(iterable, pred))
try:
result = compatible.next(iterator)
except StopIteration:
if default is not _undefined:
return default
raise LookupError('enumerable contains no matching element')
else:
try:
compatible.next(iterator)
raise LookupError('enumerable contains more than one element')
except StopIteration:
return result
def elementat(iterable, index, default=_undefined):
try:
return compatible.next(iter(skip(iterable, index)), default)
except StopIteration:
raise IndexError('enuemrable index out of range')
