theorem
  for a,b,x being Real st a>0 & a<>1 & x>0 & log(a,x) = 0 holds x = 1
proof
  let a,b,x be Real;
  assume a>0 & a<>1 & x>0 & log(a,x) = 0;
  then a to_power 0 = x by POWER:def 3;
  hence thesis by POWER:24;
end;
