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```def pfunc(inputfields, operation):
"""Parameterized computation of a new field

For example, append a field that is the sum of fields 1 and 3:
>>> z = pfunc((1, 3), operator.add)
>>> database = [(10, 25, 30, 40, 50, 60), (100, 250, 300, 10, 500, 600)]
>>> map(z, database)
[(10, 25, 30, 40, 50, 60, 65), (100, 250, 300, 10, 500, 600, 260)]
"""
def z(record):
newfield = operation(*[record[f] for f in inputfields])
return record + (newfield,)
return z

def rfunc(inputfield, operation):
"""Parameterized reduce operation

For example, find the maximum value of field 2:
>>> r = rfunc(2, max)
>>> database = [(10, 25, 30, 40, 50, 60), (100, 250, 300, 10, 500, 600)]
>>> reduce(r, database, 0)
300
"""
def z(cumval, record):
x = record[inputfield]
return operation(cumval, x)
return z

def filt_func(inputfields, operation):
"""Parameterized filter operation

For example, get records where field1 < field3:
>>> f = filt_func((1, 3), operator.lt)
>>> database = [(10, 25, 30, 40, 50, 60), (100, 250, 300, 10, 500, 600)]
>>> filter(f, database)
[(10, 25, 30, 40, 50, 60)]
"""
def z(record):
i, j = inputfields
return operation(record[i], record[j])
return z

def xfunc(fields):
"""Parameterized extract operation

For example, extract fields 1, 3, and 4
>>> x = xfunc((1,3,4))
>>> database = [(10, 25, 30, 40, 50, 60), (100, 250, 300, 10, 500, 600)]
>>> map(x, database)
[(25, 40, 50), (250, 10, 500)]
"""
def z(record):
return tuple([record[f] for f in fields])
return z

# Test the examples
if  __name__ == '__main__':
import doctest, operator
doctest.testmod()
```