reserve a, b, c, d, e for Complex;

theorem :: REAL_2'97
  a <> 0 implies a - b = a * (1 - b / a)
proof
  assume
A1: a<>0;
  thus a-b=a+-b .=a*(1+(-b)/a) by A1,Th117
    .=a*(1+-b/a) by Lm17
    .=a*(1-b/a);
end;
