reserve V for RealLinearSpace,
  u,u1,u2,v,v1,v2,w,w1,x,y for VECTOR of V,
  a,a1,a2,b,b1,b2,c1,c2,n,k1,k2 for Real;

theorem Th5:
  Gen x,y implies Ortm(x,y,u-v)=Ortm(x,y,u)-Ortm(x,y,v)
proof
  assume
A1: Gen x,y;
  hence Ortm(x,y,u-v)=Ortm(x,y,u) + Ortm(x,y,(-v)) by Th1
    .=Ortm(x,y,u) - Ortm(x,y,v) by A1,Th4;
end;
