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    "# Black boxes\n",
    "The <models> section is how a primitive block can implement a black-box BLIF primitive. Open the file 03_blackbox.xml in a text editor. in the `<models>` section, find the line: `<model name=\"multiply\"> `. It declares a black-box primitive with combinational logic that connects two inputs to one output.\n",
    "    \n",
    "Now in the `<complexblocklist>` section, find the line `<pb_type name=\"mult_16x16_slice\" height=\"4\" width=\"2\">`. This begins the definition of the black box primitive. It is given a name and dimensions in block units. The inputs and outputs are defined as two 16 bit inputs and a 32 bit output.\n",
    "\n",
    "The line:\n",
    " ```\n",
    " <pb_type name=\"mult_16x16\" blif_model=\".subckt multiply\" num_pb=\"1\">\n",
    " ```\n",
    " defines a sub-block that implements the `multiply` logic. This block also has two 16 bit inputs and a 32 bit output. Then the delays between the two inputs to the output are defined.\n",
    " \n",
    " "
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