for(;n=readline(a=[j=s=1]);print(n+':',s))for(m=i=2;i++<n&&m?1:j<n?s+=' '+(a[j++]||'_'):0;m?a[z-~m]=i:0)for(a[n-1]=2,m=x=0;x<n-1;l>m&&(z=y,m=l))for(y=x;l=x+++~y>>1,!a[x];);
Note that non-ascii characters in the above source code will be escaped (such as \x9f).