reserve i,j,k,l for Nat,
  x,x1,x2,y1,y2 for set;
reserve P,p,x,y,x1,x2 for set,
  m1,m2,m3,m4,m for marking of P,
  i,j,j1,j2,k,k1,k2,l,l1 for Nat;

theorem Th4:
  m1 c= m1 + m2
proof
  let p;
  assume p in P;
  then (m1 + m2) multitude_of p = (m1 multitude_of p)+(m2 multitude_of p)
  by Def4;
  hence thesis by NAT_1:11;
end;
