theorem Th20:
  a*(x1+x2+x3)=a*x1+a*x2+a*x3
proof
  thus a*(x1+x2+x3) = a*(x1+x2)+a*x3 by EUCLID_4:6
    .= a*x1+a*x2+a*x3 by EUCLID_4:6;
end;
