diff --git a/spec.md b/spec.md
index f6cc87a..01deb65 100644
--- a/spec.md
+++ b/spec.md
@@ -73,6 +73,7 @@ all operators:
 - `List[t] ++ List[t]`: list concatenation
 - `value :: t` (explicitly specify type of value, useful for down-casting structs, or just code readability; does not perform casting)
 - `list[index]`
+- `a => b`: lens compose
 
 ## non-nominal struct types
 ```
@@ -80,6 +81,10 @@ all operators:
 type User = { name: List[Char] }
 type DbUser = { name: List[Char], pass: List[Char] }
 
+def example4(u: User) : List[Char] {
+  u:name    # colon is used to access fields
+}
+
 def example(u: User) : DbUser {
   u with pass: "1234"
   # has type { name: List[Char], pass: List[Char] }
@@ -245,18 +250,18 @@ Export signature file `List.li`:
 type List[t] = 'End | 'Cons {head:t, tail:List[t]}
 
 # not providing a function body makes it a function signature definition
-def [t] `a++b`(a: List[t], b: List[t]) -> List[t];
+def [t] `a++b`(a: List[t], b: List[t]) -> List[t]
 
 def [t] Match.`a++b`(
   value: List[t],
   l: List[t],
   r: MatchUtil.Var[List[t]]
-) -> Option[{ r: List[t] }];
+) -> Option[{ r: List[t] }]
 ```
 
 Import signature file `Option.li`:
 ```
-type Option[t] = 'None | 'Some t;
+type Option[t] = 'None | 'Some t
 ```
 
 Compilation unit `List.lu`:
@@ -279,4 +284,87 @@ Each compilation unit gets compiled to implementation-specific bytecode.
 
 Templated functions can only be compiled partially during a compilation unit. This will impact compile speeds.
 
-Avoid templated functions wherever possible.
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+Avoid templated functions wherever possible.
+
+## Hide record fields in module signatures
+Signatue:
+```
+type User = {name: List[Char], ...}
+  # the ... is used to indicate that this is a partial type definition
+  # User, as given here, can not be constructed, but name can be accessed
+```
+Compilation unit:
+```
+type User = {name: List[Char], password: List[Char]}
+```
+
+## Hide union variants in module signatures
+Signature:
+```
+type DType = 'Int | 'UInt | 'Byte | ...
+  # users of this can never do exhaustive pattern matching on this
+```
+Compilation unit:
+```
+type DType = 'Int | 'UInt | 'Byte
+  # this compilation unit can actually do exhaustive pattern matching on this
+```
+
+## Note on hidden union variants / record fields
+To make these work, the following is legal:
+```
+def example() -> 'Int | 'UInt | ...
+
+def example() -> 'Int | 'UInt | 'Byte | 'Char {
+  # ...
+}
+```
+
+## Extensible unions
+```
+type Plan = dyn 'Plan;
+              # ^ this is not a union variant tag, but a quoted identifier
+
+extend Plan with 'ReadlnPlan
+extend Plan with 'WritelnPlan
+
+# pattern matching against these is always non-exhaustive.
+# can only pattern match with the imported extensions
+
+```
+
+## Lenses
+In the stdlib:
+```
+type Lens[t,f] = {get: t -> f, set: t -> f -> t}
+
+def [a,b,c] `a=>b`(x: Lens[a,b], y: Lens[b,c]) -> Lens[a,c] {
+  {
+    get: t:a -> y:get(x:get(t)),
+    set: t:a -> f:c -> x:set(t, y:set(x:get(t), f))
+  }
+}
+```
+
+Since `a:f1:f2` is the field access syntax, the lens creation syntax is similar: `&Type:field1:field2`
+
+So you can do:
+```
+type Header = {text: String, x: Num}
+type Meta = {header: Header, name: String}
+
+&Header:header:text (myHeader, "new meta:header:text value")
+# which is identical to:
+(&Header:header => &Meta:text) (myHeader, "new meta:header:text value")
+```
+
+However, it is cleaner to use `with`:
+```
+myHeader with header:text: "new meta:header:text value"
+```
+
+
+## Pure IO
+```
+TODO  
+```
\ No newline at end of file