Restore notes on portability

src/SUMMARY.md

diff --git a/src/SUMMARY.md b/src/SUMMARY.md
index 0ee7ea1..bf111bb 100644
--- a/src/SUMMARY.md
+++ b/src/SUMMARY.md
@@ -32,6 +32,7 @@
 - [Static Guarantees](./static-guarantees/static-guarantees.md)
     <!-- TODO: Define Sections -->
 - [Portability](./portability/portability.md)
+    - [The Trait System](./portability/traits.md)
     <!-- TODO: Define more sections -->
 - [Concurrency](./concurrency/concurrency.md)
     <!-- TODO: Define Sections -->

src/portability/portability.md

diff --git a/src/portability/portability.md b/src/portability/portability.md
index fec7a58..26fb9c2 100644
--- a/src/portability/portability.md
+++ b/src/portability/portability.md
@@ -1,3 +1,14 @@
 # Portability
 
-> ❌: This section has not yet been written. Please refer to [rust-embedded/book#6](https://github.com/rust-embedded/book/issues/6) for discussion of this section.
+> ❌: This section has not yet been written. Please refer to [embedded-wg#119](https://github.com/rust-lang-nursery/embedded-wg/issues/119) for discussion of this section.
+
+## Building something bigger
+
+> drivers that work for more than one chip
+
+![](./../assets/embedded-hal.svg)
+
+* Device Crates (one per chip)
+* HAL Implementation Crates (one per chip)
+* `embedded-hal` (only one)
+* Driver Crates (one per external component)

src/portability/traits.md

diff --git a/src/portability/traits.md b/src/portability/traits.md
new file mode 100644
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@@ -0,0 +1,50 @@
+# The Trait System
+
+```rust
+/// Single digital push-pull output pin
+pub trait OutputPin {
+    /// Drives the pin low
+    fn set_low(&mut self);
+
+    /// Drives the pin high
+    fn set_high(&mut self);
+}
+```
+
+[rust-embedded/embedded-hal](https://github.com/rust-embedded/embedded-hal/blob/master/src/digital.rs)
+
+```rust
+impl<MODE> OutputPin for OutputGpio<MODE> {
+    fn set_low(&mut self) {
+        self.set_pin_low()
+    }
+
+    fn set_high(&mut self) {
+        self.set_pin_high()
+    }
+}
+```
+
+> this goes in your chip crate
+
+
+```rust
+impl<SPI, CS, E> L3gd20<SPI, CS>
+where
+    SPI: Transfer<u8, Error = E> + Write<u8, Error = E>,
+    CS: OutputPin,
+{
+    /// Creates a new driver from a SPI peripheral
+    /// and a NCS (active low chip select) pin
+    pub fn new(spi: SPI, cs: CS) -> Result<Self, E> {
+        // ...
+    }
+    // ...
+}
+```
+
+[japaric/l3gd20](https://github.com/japaric/l3gd20/blob/master/src/lib.rs)
+
+## N\*M >>> N+M
+
+> to re-use a driver, just implement the embedded-hal interface
\ No newline at end of file