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```from __future__ import generators, nested_scopes

def itercat(*iterators):
"""Concatenate several iterators into one."""
for i in iterators:
for x in i:
yield x

def iterwhile(func, iterator):
"""Iterate for as long as func(value) returns true."""
iterator = iter(iterator)
while 1:
next = iterator.next()
if not func(next):
raise StopIteration
yield next

def iterfirst(iterator, count=1):
"""Iterate through 'count' first values."""
iterator = iter(iterator)
for i in xrange(count):
yield iterator.next()

def iterstep(iterator, n):
"""Iterate every nth value."""
iterator = iter(iterator)
while 1:
yield iterator.next()
# skip n-1 values
for dummy in range(n-1):
iterator.next()

def itergroup(iterator, count):
"""Iterate in groups of 'count' values. If there
aren't enough values, the last result is padded with
None."""
iterator = iter(iterator)
values_left = [1]
def values():
values_left[0] = 0
for x in range(count):
try:
yield iterator.next()
values_left[0] = 1
except StopIteration:
yield None
while 1:
value = tuple(values())
if not values_left[0]:
raise StopIteration
yield value

def xzip(*iterators):
"""Iterative version of builtin 'zip'."""
iterators = map(iter, iterators)
while 1:
yield tuple([x.next() for x in iterators])

def xmap(func, *iterators):
"""Iterative version of builtin 'map'."""
iterators = map(iter, iterators)
values_left = [1]
def values():
# Emulate map behaviour, i.e. shorter
# sequences are padded with None when
# they run out of values.
values_left[0] = 0
for i in range(len(iterators)):
iterator = iterators[i]
if iterator is None:
yield None
else:
try:
yield iterator.next()
values_left[0] = 1
except StopIteration:
iterators[i] = None
yield None
while 1:
args = tuple(values())
if not values_left[0]:
raise StopIteration
yield func(*args)

def xfilter(func, iterator):
"""Iterative version of builtin 'filter'."""
iterator = iter(iterator)
while 1:
next = iterator.next()
if func(next):
yield next

def xreduce(func, iterator, default=None):
"""Iterative version of builtin 'reduce'."""
iterator = iter(iterator)
try:
prev = iterator.next()
except StopIteration:
return default
single = 1
for next in iterator:
single = 0
prev = func(prev, next)
if single:
return func(prev, default)
return prev
```

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