postfix to infix by kinaba

#!ruby -pl
x=proc{|p|c=19&86%$_[~/.$/];chop;c<4?(p>c&2?"(%s)":"%s")%("%s"+$&+x[c])%x[c&2]:$&}
$_=x[0]

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

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