theorem Th11:
  Gen x,y implies Orte(x,y,u-v)=Orte(x,y,u)-Orte(x,y,v)
proof
  assume
A1: Gen x,y;
  hence Orte(x,y,u-v)=Orte(x,y,u) + Orte(x,y,(-v)) by Th10
    .=Orte(x,y,u) - Orte(x,y,v) by A1,Th9;
end;
