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PAICE's evaluation statistics for stemming algorithms
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Given a list of words with their real lemmas and stems according to stemming algorithm under evaluation,
counts Understemming Index (UI), Overstemming Index (OI), Stemming Weight (SW) and Error-rate relative to truncation (ERRT).

   >>> from nltk.metrics import Paice

  
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Understemming and Overstemming values
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    >>> lemmas = {'kneel': ['kneel', 'knelt'],
    ...           'range': ['range', 'ranged'],
    ...           'ring': ['ring', 'rang', 'rung']}
    >>> stems = {'kneel': ['kneel'],
    ...          'knelt': ['knelt'],
    ...          'rang': ['rang', 'range', 'ranged'],
    ...          'ring': ['ring'],
    ...          'rung': ['rung']}
    >>> p = Paice(lemmas, stems)
    >>> p.gumt, p.gdmt, p.gwmt, p.gdnt
    (4.0, 5.0, 2.0, 16.0)

    >>> p.ui, p.oi, p.sw
    (0.8, 0.125, 0.15625)

    >>> p.errt
    1.0

    >>> p.coords
    [(0.0, 1.0), (0.0, 0.375), (0.6, 0.125), (0.8, 0.125)]
	