Understand double entry system.

Before understanding double entry system, let us understand traditional
single entry system: 

  - All incomes are positive (I)
  - All assets cash, bank balance are positive (A)
  - All receivables are positive (R)
  - All expenses are negative (E)
  - All payables are negative (P)

Total Income + Receivables =~ Total Expenses + Payables

Accumulated(Income) (Dr)                       + Receivables (Dr) =~ 
Accumulated(Expenses) (Dr) + Cash And Bank Assets (Dr)  + Payables (Cr)

Expenses  = Expenses for Service + Expenses For Assets (e.g. Computer)

Note: We don't directly track which expenses are for service and
      which are for assets.
      Main source of income is maintenance. Also includes rental income.

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In double entry system, we try to balance the figures at any time.

T-Zero :    No Income, No expenses, all zero.

         (Dr)           (Cr)
T-1 : Receivables  =~  Payables      : We expect to earn 10L, spend 10L
                                     Payables may include provisioning. 

T-2 : We get some maintenance income.
                  (Dr)                      (Cr)
      Receivables + Cash + Bank-Balance =~ Payables


T-3 : We spend some money for Service.
                     (Dr)                                  (Cr)
      Receivables + TotalExpense + Cash + Bank-Balance =~ Payables 

      Note: As expense goes high, cash comes down.
            Very important point to note that why Expense is Dr, not Cr.

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