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 Th7:
  (m1 + m2) - m2 = m1
proof
  let p be object;
  assume
A1: p in P;
  m2 c= (m1 + m2) by Th4;
  hence ((m1 + m2) - m2).p = (m1 + m2) .p - m2.p by A1,Def5
    .= m1.p + m2.p - m2.p by A1,Def4
    .= m1.p;
end;
