Starting on Step12

Step11/README.md

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 `ConcatStreams` reads and outputs all of its first input, followed by the second input, and so on.  If `ConcatStreams` could only handle, say, two input streams, it might call them `IN1` and `IN2`, but it can handle any number, so it uses something called an "array port", and the individual connection points are labelled using an indexing notation, e.g. `IN[0]`, `IN[1]`, and so on.  
 
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Step12/README.md

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+<link rel="stylesheet" type="text/css" href="../style.css">
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+## Step12-Subnets
+
+As we stated above, FBP has a concept called "subnets", which may be thought of as ordinary FBP networks, with "sticky" connections.  In FBP we like to go from a high-level design to progressively lower levels, until we are ready to start filling in component names. These lower-level networks are called "subnets" - basically diagrams with "sticky" connections, that can both function as components but themselves comprise components - or still lower-level subnets - so a complex application may be structured as a "tree" of subnets, where the root is a high-level network with all or most of the nodes being subnets.
+
+DrawFBP has a facility called "Excise" where the designer marks off a section of the diagram and "excises" it, forming a subnet and replacing the excised subnet with a single block that instantiates the subnet. The sticky ends are called "External Ports", and form the "bridge" between the indide and the outside of a subnet.
+
+Let us go back to the diagram we show in <a href="">Step05-Choose components and set values for other parts</a>:
+
+![Diagram with components and IIPs filled in](../Step05/Step05.png)
+
+Now let us suppose we want to hid the "Ignore" node - we can "excise" the middle two components and put them in a separate subnet called, say, "Filter".  
+
+The way we do this is to create an instance of the "Enclosure" block type, and *stretch* it to surround the blocks we want to excise - mouse over the corners, and drag them to where you want them to be.  Here is the diagram with the Enclsure added.
+
+![Diagram with "stretched" Enclosure](Step12.png)
+
+Now the only part of the Enclosure which you can click on to get the action list is the coloured section at the top.
+
+
+
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