Калькулятор для расчета родства

<input type="button" value="p" onclick="a='p'+a; fn()">
<input type="button" value="P" onclick="a='P'+a; fn()">
 
<input type="button" value="b" onclick="a='b'+a; fn()">
<input type="button" value="B" onclick="a='B'+a; fn()">
 
<input type="button" value="s" onclick="a='s'+a; fn()">
<input type="button" value="S" onclick="a='S'+a; fn()">
 
<input type="button" value="q" onclick="a='q'+a; fn()">
<input type="button" value="Q" onclick="a='Q'+a; fn()">
 
<input type="button" style="float:right" value="clr" onclick="a=''; fn()">