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;
reserve RAS for MidSp-like non empty ReperAlgebraStr over n+2;
reserve a,b,d,pii,p9i for Point of RAS;
reserve p,q for Tuple of (n+1),RAS;
reserve m for Nat of n;

theorem Th7:
  i is Nat of n iff i in Seg(n+1)
proof
  i is Nat of n iff 1<=i & i<=n+1 by Def2;
  hence thesis by FINSEQ_1:1;
end;
