reserve k,m,n for Element of NAT,
  a,X,Y for set,
  D,D1,D2 for non empty set;
reserve p,q for FinSequence of NAT;
reserve x,y,z,t for Variable;
reserve F,F1,G,G1,H,H1 for ZF-formula;

theorem Th19:
  H is being_membership implies H.1 = 1
proof
  assume H is being_membership;
  then consider x,y such that
A1: H = x 'in' y;
  H = <* 1,x,y *> by A1,FINSEQ_1:def 10;
  hence thesis;
end;
