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;
