'Return True if this Feature always returns True when other does
More precisely, return True if this feature refers to the same property as other;
and this Feature looks at all positions that other does (and possibly
other positions in addition).
#For instance, importing a concrete subclass (Feature is abstract)
>>> from nltk.tag.brill import Word, Pos
>>> Word([-3,-2,-1]).issuperset(Word([-3,-2]))
True
>>> Word([-3,-2,-1]).issuperset(Word([-3,-2, 0]))
False
#Feature subclasses must agree
>>> Word([-3,-2,-1]).issuperset(Pos([-3,-2]))
False
:param other: feature with which to compare
:type other: (subclass of) Feature
:return: True if this feature is superset, otherwise False
:rtype: bool'

def issuperset(self, other):

    return ((self.__class__ is other.__class__) and (set(self.positions) >= set(other.positions)))
