What can be inferred from the following premises in a single inference step (ignoring inferences that add new predicates or constants)? For all x_12, if x_12 writes a letter, then x_12 is not taking a plane. For all x_12, x_12 will take a plane. Jessica is happy if and only if Richard is writing a letter.
If Jessica were happy, then Richard is writing a letter. If Richard writes a letter, then Jessica is happy. For all x_12, x_12 will not write a letter.