reserve
  a for natural Number,
  k,l,m,n,k1,b,c,i for Nat,
  x,y,z,y1,y2 for object,
  X,Y for set,
  f,g for Function;
reserve p,q,r,s,t for FinSequence;
reserve D for set;

theorem
  p <> {} iff len p >= 1
proof
  hereby
    assume p <> {};
    then len p+1 > 0+1 by XREAL_1:8;
    hence len p >=1 by NAT_1:13;
  end;
  assume len p >= 1;
  hence thesis;
end;
