Aard custom (#49)

* Merge changes to docs

* Fix typo

* Correct SUMMARY so it compiles; update .gitignore

* Clean up statements.md

Make syntax and notation consistent with Rust source code.

* Fix statements for Merkle trees and compound types

* First draft of custom statements and small updates to signedpod.md

* Update book/src/merkletree.md

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* merklestatements correct typo

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* add todo for gadget ids

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>

* Separate out custom statements version 1

* More details on custom statements version 1

* new file custom2

* Partial draft of version 2

* First draft of version 2 spec, it's kind of a mess

* Another version of the custom predicates spec

* Update book/src/custom2.md

Co-authored-by: Eduard S. <eduardsanou@posteo.net>

* Simple example of deduction rule applied in circuit

* Implement Edu's comments on custom predicates

* Backend predicates must be defined in groups

* Add more examples

* Two diff statements using same constant

* Remove deprecated example

---------

Co-authored-by: Ahmad Afuni <root@ahmadafuni.com>
Co-authored-by: Eduard S. <eduardsanou@posteo.net>

book/src/SUMMARY.md

@@ -3,6 +3,7 @@
 - [Introduction](./introduction.md)
 
 # Specification
+- [Front and back end](./front_and_back.md)
 - [The frontend structure of a POD]()
   - [Frontend POD value types](./values.md)
   - [Anchored keys](./anchoredkeys.md)
@@ -13,6 +14,11 @@
   - [Statements](./statements.md)
     - [Statements involving compound types and Merkle trees](./merklestatements.md)
   - [Operations](./operations.md)
+    - [Simple example](./simpleexample.md)
+  - [Custom statements and custom operations](./custom.md)
+    - [Version 1](./custom1.md)
+    - [Version 1, written again](./custom1b.md)
+    - [Custom statement example](./customexample.md)
 - [POD types](./podtypes.md)
   - [SignedPOD](./signedpod.md)
   - [MainPOD](./mainpod.md)