theorem Th17:
  Sum<* v,v *> = (1r+1r) * v
proof
  thus Sum<* v,v *> = v + v by RLVECT_1:45
    .= 1r * v + v by Def5
    .= 1r * v + 1r * v by Def5
    .= (1r+1r) * v by Def3;
end;
