reserve n for Nat,
  i for Integer,
  p, x, x0, y for Real,
  q for Rational,
  f for PartFunc of REAL,REAL;

theorem Th13:
  log(number_e,exp_R.x) = x
proof
  thus log(number_e,exp_R.x) =log(number_e,exp_R(x)) by SIN_COS:def 23
    .=x by Th12;
end;
