Txn sub-groups

[hrj]

Comment by @jaa127 via email:

It would be cool to be able generate transactions like these:

2015-01-02   tmx 18/45 D12L
        {
                Gas:He:Tank:102         -(273 - 243) ;  40 bars
                Gas:He:Tank:103         -(100 - 70) ; 30 bars
                Gas:He:Tank:104         -(80  - 50) ; 30 bars
                Divers:jaa127:He ; <== No amount here!
        }
        {
                Gas:O2:Tank:208         -(180 - 171) ; 9 bars
                Gas:O2:Tank:209         -(143 - 134) ; 9 bars
                Divers:jaa127:O2  ; <== No amount here!
        }

Because in real life it is "single transaction", but with current
mechanism, it has to be either:

• all postings has to be calculated (and then there is change to
  miss-calculate gas amounts by type, and mix up gas types. E.g. on
  Abandon "bars" will zero out, but credit/debit by gas type is wrong. If
  that happens, then it will badly mess up billing because Helium is 5-10x
  more expensive than Oxygen in Finland.
• Or there has to be two separate transactions, which isn't so nice. For
  diver who is mixing gases it would be really nice to have one clean and
  clear single transaction, which is listing all gases what you used.

So scopes looks like perfect solution for that!

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PS. This was clubbed with the scopes idea, but I think it is better to give it a separate name, say, "transaction sub-group".