RNAi in GIL
===========
GIL will automatically generate a siRNA for RNAi using the ``Block`` keyword. GIL will attempt to generate 
a siRNA which blocks specific genes while ignoring others. Start by putting any GIL code into the curly 
brackets. To add gene to be blocked, use ``&``. To ignore a gene, use ``!``. 

Example:

.. code-block:: none

   Block
   {
       SomeGene
       &AnotherGene
       !NotThisGene
   }

To change the length of the siRNAs generated by Block, use ``#SetAtr RNAI_Len:length``