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;
reserve W for ATLAS of RAS;
reserve v for Vector of W;
reserve x,y for Tuple of (n+1),W;

theorem Th20:
  1 in Seg(n+1)
proof
  0 <= n by NAT_1:2;
  then 0+1 <= n+1 by XREAL_1:7;
  hence thesis by FINSEQ_1:1;
end;
