postfix to infix by nn

function f(p,q,c){c=$NF
NF--
return(q=index("+-*/",c))&&(c=c f(q))(c=f(q=q>2?3:1)c)&&p>q?"("c")":c}BEGIN{FS=_}$0=f()

Note that non-ascii characters in the above source code will be escaped (such as \x9f).

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