add more complex fold example
add more complex fold example

jcrist/dask,wiso/dask,PhE/dask,clarkfitzg/dask,pombredanne/dask,mraspaud/dask,PhE/dask,ContinuumIO/dask,mrocklin/dask,gameduell/dask,blaze/dask,mraspaud/dask,jakirkham/dask,jakirkham/dask,ssanderson/dask,clarkfitzg/dask,cpcloud/dask,vikhyat/dask,chrisbarber/dask,vikhyat/dask,wiso/dask,cowlicks/dask,pombredanne/dask,dask/dask,ContinuumIO/dask,jcrist/dask,blaze/dask,mrocklin/dask,dask/dask,ssanderson/dask,mikegraham/dask