
theorem th7:
  for p,q,r be boolean object holds
    (p '&' q '&' r) <=> (p '&' (q '&' r)) = TRUE
  proof
    let p,q,r be boolean object;
A1: p = TRUE or p = FALSE by XBOOLEAN:def 3;
A2: q = TRUE or q = FALSE by XBOOLEAN:def 3;
    r= TRUE or r = FALSE by XBOOLEAN:def 3;
    hence thesis by A1,A2;
  end;
