Builder outputs are now a sort of subgraph

Hello sho, 

Please review the following commits I made in branch 'jeremy-conditionals'.

b073c0b4f47bafdaa1e33619adc0c4815e3b1cf1 (2013-02-16 18:08:35 -0800)
Doc blocks

9a144d706334d645d0a98930a6ad5a16451dae1c (2013-02-16 18:04:29 -0800)
Builder outputs == subgraphs

fe2ae9f36079fcb6e2a359f82bd7f28abd8e1f7d (2013-02-16 18:03:11 -0800)
Builder outputs == subgraphs

R=sho