The correct answer to this open question is the following.
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The police officer is referring to common law.
Although Susie has just moved from the countryside into town, she has to have in mind that in the city, where millions of people live, there are many different customs and ways of being and behave.
n the country, she could do other things. But now, in the city, her pet sheep needs to be leashed because otherwise, it could represent a risk for people or property.
That is why she was stopped one day as she and Toby are walking through the park by a local police officer. The officer told Susie that she's breaking the law by walking Toby without a leash. The police officer was appealing to the common law that says that in the city, these animals must be leashed in public places or kept at home.
