Make parameter to nth function implicit
Make cons operator right associative
Huge rewrite

- chain construction now uses incidence proof as the term. The elements
can be inferred form it anyway.
- Include length of the chain in the datatype definition
- Made \in and irred into their own inductive datatypes
- Use inductive type to enforce incidence condition on Incidence
Planes. This forces us to make the incidence proof object in the chain
constructor relevant.