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Map the enumerable to the incremental state of a calculation by
applying the given block, in turn, to each element and the
previous state of the calculation, resulting in an enumerable of
the state after each iteration.

@return [Enumerable] Generate a stream of intermediate states
  resulting from applying a binary operator. Equivalent to a
  stream of +#inject+ calls on first N elements, increasing N from
  zero to the size of the stream. NOTE: Preserves laziness if
  +enumerable+ is lazy.

@param initial [Object] Initial state value to yield.

@yield [memo, obj] Invokes given block with previous state value
  +memo+ and next element of the stream +obj+.

@yieldreturn [Object] The next state value for +memo+.
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