Add example of how to use multiple values in a sack

[krlawrence]

Gremlin currently does not provide a way for a sack to contain multiple values. You can do it using a map, but it gets a bit messy. It would be good to have an example in the book that shows how to work around this. Such as, given the following graph:

g.addV('stop').property(id,'s1').as('s1').
  addV('stop').property(id,'s2').as('s2').
  addV('stop').property(id,'s3').as('s3').  
  addV('stop').property(id,'s4').as('s4').  
  addV('stop').property(id,'s5').as('s5').  
  addV('stop').property(id,'s6').as('s6').  
  addV('stop').property(id,'s7').as('s7').
  addE('link').from('s1').to('s2').property('p1',1).property('p2',2).
  addE('link').from('s2').to('s3').property('p1',3).property('p2',4).
  addE('link').from('s3').to('s4').property('p1',5).property('p2',6).
  addE('link').from('s4').to('s5').property('p1',7).property('p2',8).
  addE('link').from('s5').to('s6').property('p1',9).property('p2',10).
  addE('link').from('s6').to('s7').property('p1',11).property('p2',12)

We could work with two sets of edge properties as follows

g.withSack(['a':0,'b':0]).
  V('s1').
  repeat(
    outE().as('x').
    sack(assign).
      by(project('a','b').
        by(sack().math('a + _').by().by(select('x').values('p1'))).
        by(sack().math('b + _').by().by(select('x').values('p2')))).
    inV()).
    until(not(out())).
  sack()

Which produces

{'a': 36.0, 'b': 42.0}

[krlawrence]

Other examples could include complex list manipulation such as

gremlin> g.V(1,2,3).values('city').fold().sack(assign)
==>[Atlanta,Anchorage,Austin]

gremlin> g.V(1,2,3).values('city').fold().sack(assign).sack()
==>[Atlanta,Anchorage,Austin]

gremlin> g.V(1,2,3).values('city').fold().sack(assign).V(4).union(values('city'),sack().unfold()).fold().sac
k(assign).sack()
==>[Nashville,Atlanta,Anchorage,Austin]