Acrostic polyglot programming
Brainfuck -&gt; Ruby
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Horizontal is this little gem:
,[.&gt;+[-&lt;,----------[&gt;+&lt;[-]]&gt;],]

And Vertical is this:
eval&quot;a=[i=0]*3e4;&quot;+$&lt;.bytes.map{|b|{?.,&quot;putc a[i]&quot;,?,,&quot;a[i]=getc&quot;,?[,&quot;while a[i]&gt;0&quot;,?],&quot;end&quot;,?&lt;,&quot;i-=1&quot;,?&gt;,&quot;i+=1&quot;,?+,&quot;a[i]+=1&quot;,?-,&quot;a[i]-=1&quot;}[b]}*&quot;;&quot;

Which is a brainfuck interpreter stolen from this question. It qualifies for all of the hard mode criteria except the no comments bit, which I don't think is really fair to apply to brainfuck, or pretty much nothing but BF-&gt;BF would be possible.
Horizontal program runs till EOF(0) is read. Assumes newline at end of file.