Transaction and Input:
- Finalise specification of diamond-graph transactions solution
- Document Input..source_transactions_add   ( transaction_id, source )
- Document Input..source_transactions_remove( transaction_id, source )
- Document Input..source_transactions_remove_source( source )
- Finalize and document source_transactions_remove_source( output )
- Call this.source_transactions_remove_source( output ) on Input._remove_source()