John and joan enter a contract whereby john will feed joan's cat for $100 while joan is on vacation. another part of the contract provides that john will mow joan's whole yard for the summer for $500. just before joan leaves for vacation, john discovers that joan's cat is not a domesticated cat but is instead a lion rescued from the circus. in the area in which john and joan live, it is illegal to keep or care for a wild animal such as a lion in a domestic setting. john tells joan to forget it, that she misrepresented the nature of the animal, and that she can forget about the contract, including the lawn mowing deal. joan, being a somewhat reasonable person, feels bad about not telling john the whole truth about the cat, and she is willing to compromise on that issue. she does, however, want to enforce the yard mowing agreement. what is her best argument?