∃ c. p(a, c) ∧ rtc(m, c, b) ⊦ ∃ c. rtc(m, a, c) ∧ p(c, b)
∃ c. rtc(m, a, c) ∧ p(c, b) ⊦ ∃ c. p(a, c) ∧ rtc(m, c, b)