class_docstr: Instead of marching the sub-matchers along in parallel, this matcher
  pre-reads the scores for a large block of documents at a time from each
  matcher, accumulating the scores in an array.
  
  This is faster than the implementation using a binary tree of
  :class:`~whoosh.matching.binary.UnionMatcher` objects (possibly just
  because of less overhead), but it doesn't allow getting information about
  the "current" document other than the score, because there isn't really a
  current document, just an array of scores.
code: def _min_id(self):
      active = [subm for subm in self._submatchers if subm.is_active()]
      if active:
          return min(subm.id() for subm in active)
      else:
          return self._doccount
  
imports: from __future__ import division
  from array import array
  from whoosh.compat import xrange
  from whoosh.matching import mcore
label_desc: Use image node_id 3 for calling the ArrayUnionMatcher obj's underlying member method code with example usage: obj._min_id() and returns: min, self