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 Th11:
  H is atomic implies len H = 3
proof
A1: now
    assume H is being_equality;
    then consider x,y such that
A2: H = x '=' y;
    H = <* 0,x,y *> by A2,FINSEQ_1:def 10;
    hence thesis by FINSEQ_1:45;
  end;
A3: now
    assume H is being_membership;
    then consider x,y such that
A4: H = x 'in' y;
    H = <* 1,x,y *> by A4,FINSEQ_1:def 10;
    hence thesis by FINSEQ_1:45;
  end;
  assume H is atomic;
  hence thesis by A1,A3;
end;
