A training tuple τ = t 1 , R , t 2 is considered "similar" to τ if R = R , one of the terms matches exactly, and the other term has the same head word.
(R = R ) ∧ (t 1 = t 1 ) ∧ (head (t 2 ) = head (t 2 )), or (R = R ) ∧ (t 2 = t 2 ) ∧ (head (t 1 ) = head (t 1 )).