5-1

# Call the each method of each collection in turn.
# This is not a parallel iteration and does not require enumerators.
def sequence(*enumerables, &block)
  enumerables.each do |enumerable|
    enumerable.each(&block)
  end
end

# Iterate the specified collections, interleaving their elements.
# This can't be done efficiently without external iterators.
# Note the use of the uncommon else clause in begin/rescue.
def interleave(*enumerables)
  # Convert enumerable collections to an array of enumerators.
  enumerators = enumerables.map {|e| e.to_enum }
  # Loop until we don't have any more enumerators.
  until enumerators.empty?
    begin
      e = enumerators.shift   # Take the first enumerator
      yield e.next            # Get its next and pass to the block
    rescue StopIteration      # If no more elements, do nothing
    else                      # If no exception occurred
      enumerators << e        # Put the enumerator back
    end
  end
end

# Iterate the specified collections, yielding tuples of values,
# one value from each of the collections. See also Enumerable.zip.
def bundle(*enumerables)
  enumerators = enumerables.map {|e| e.to_enum }
  loop { yield enumerators.map {|e| e.next} }
end

# Examples of how these iterator methods work
a,b,c = [1,2,3], 4..6, 'a'..'e'
sequence(a,b,c) {|x| print x}   # prints "123456abcde"
interleave(a,b,c) {|x| print x} # prints "14a25b36cde"
bundle(a,b,c) {|x| print x}     # '[1, 4, "a"][2, 5, "b"][3, 6, "c"]'