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;
