Write the following argument in symbolic form. 

If Trevor eats a salad, he needs salad dressing. If he eats a sandwich, he needs bread. Trevor will eat either a salad or a sandwich for lunch. If he needs salad dressing or bread, then he must go to the grocery store. Therefore, Trevor must go to the grocery store before lunch. 

Prove or disprove the argument you wrote above. To prove, provide an algebraic proof with reasons for each step. To disprove, provide a counterexample (assignment of truth values to variables that cause the arguments to be false).