What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? Barbara is writing a letter. If Barbara writes a letter, then Thomas is driving a car. Jessica is not a doctor. Jessica is a doctor if and only if David is a mechanic.
Thomas is driving a car. Barbara is writing a letter and Jessica is not a doctor. Jessica is not a doctor and Barbara is writing a letter. If Jessica were a doctor, then David is a mechanic. If David were a mechanic, then Jessica is a doctor. David is not a mechanic.