title Zúzalékszóró fej használható-e

participant SnowPlow
participant GravelHead

[->SnowPlow:
activate SnowPlow
SnowPlow->GravelHead: canBeUsedBy(SnowPlow)
activate GravelHead

GravelHead->SnowPlow: getGravelCount()
activate SnowPlow
SnowPlow-->GravelHead: gravelCount
deactivate SnowPlow

alt gravelCount > 0
    GravelHead-->SnowPlow: true
else
    GravelHead-->SnowPlow: false
end

deactivate GravelHead

[<--SnowPlow:
deactivate SnowPlow