theorem
  for p, q being Path of t1,t2 st p = pr2 l1 & q = pr2 l2 & l1,l2
  are_homotopic holds pr2 H is Homotopy of p,q
proof
  let p, q be Path of t1,t2 such that
A1: p = pr2 l1 & q = pr2 l2 & l1,l2 are_homotopic;
  thus p,q are_homotopic
  proof
    take pr2 H;
    thus thesis by A1,Lm4;
  end;
  thus thesis by A1,Lm4;
end;
