Implements the system operation
bnk payBills: billCollection.
(The bank argument is redundant; all bills are paid by transferring money between accounts in the same bank. Needs cleaning up.)
The example illustrates that a RoleMethod calls another Context as a subroutine.
(BillPayer::payBills calls the BankTransfer context to pay one bill at the time).
The code can be read and run in BabyIDE. Informally, the code can be read at
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