reserve p,q,r for FinSequence;
reserve u,v,x,y,y1,y2,z for object, A,D,X,Y for set;
reserve i,j,k,l,m,n for Nat;

theorem Th24:
  x in dom p implies x <> 0
proof
  assume x in dom p;
  then x in Seg(len p) by FINSEQ_1:def 3;
  hence thesis by FINSEQ_1:1;
end;
