Substitutivity of Equivalence Let A, m and n be wffs and let an be the result of replacing M by N at zero or more occurrences Henceforth called designate occurrences )of M in A 1. AN is a wff 2. IfFM=N then FA=An (≡8b) If H M=n then F A=An Logic in Computer Science - p. 15/17Substitutivity of Equivalence Let A,M and N be wffs and let AMN be the result of replacing M by N at zero or more occurrences (henceforth called designate occurrences) of M in A. 1. AMN is a wff. 2. If |= M ≡ N then |= A ≡ AMN . (≡ sub) If ` M ≡ N then ` A ≡ AMN Logic in Computer Science – p.15/17