theorem Th65:
  for p being PartState of S, k being Nat
   holds IC DecIC (p,k) = IC p -' k
proof
  let p be PartState of S, k be Nat;
   IC S in dom Start-At(IC p-'k,S) by TARSKI:def 1;
  hence IC DecIC (p,k) = (Start-At((IC p)-'k,S)).IC S by FUNCT_4:13
    .= IC p -' k by FUNCOP_1:72;
end;
