theorem Th72:
  for f,g being Function, A being set st A misses dom g holds
  (f +* g)|A = f|A
proof
  let f,g be Function, A be set;
  assume A misses dom g;
  then dom g /\ A = {};
  then dom (g|A) = {} by RELAT_1:61;
  then g|A = {};
  hence (f +* g)|A = f|A +* {} by Th71
    .= f|A;
end;
