The next test case shows that we can look up pay rates even if the pay rate changes:

NavigationTest>>testPayLastMonth | contract firstVersion secondVersion | contract := Contract new. firstVersion := ContractVersion pay: 1000. contract versionAt: '1/1/99' asDate put: firstVersion. secondVersion := ContractVersion pay: 2000. contract versionAt: '2/1/99' asDate put: secondVersion. self assert: (contract versionAt: '1/1/99' asDate) pay = 1000. self assert: (contract versionAt: '2/1/99' asDate) pay = 2000

For now we can use simple Dates as the lookup key.

This test case is already satisfied by the code above.