reserve n,i,j,k,l for Nat;
reserve D for non empty set;
reserve c,d for Element of D;
reserve p,q,q9,r for FinSequence of D;

theorem Th4:
  l<=j or l=j+1 or j+2<=l
proof
A1: j+1+1 = j+2;
  l<j+1 or l = j+1 or j+1<l by XXREAL_0:1;
  hence thesis by A1,NAT_1:13;
end;
