Added FSet3 with a formalisation of finite sets based on Michael's one.
Higher Priority
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- if the constant definition gives the wrong definition
  term, one gets a cryptic message about get_fun
- have FSet.thy to have a simple infrastructure for 
  finite sets (syntax should be \<lbrace> \<rbrace>,
  look at Set.thy how syntax is been introduced)
- think about what happens if things go wrong (like
  theorem cannot be lifted) / proper diagnostic 
  messages for the user
- Handle theorems that include Ball/Bex
- quotient_respects and preserves in a natural form.
Lower Priority
==============
- inductions from the datatype package have a strange
  order of quantifiers in assumptions.
- wrapper that translates an an original theorem given
  a list of quotient_types as an attribute
- find clean ways how to write down the "mathematical"
  procedure for a possible submission (Peter submitted 
  his work only to TPHOLs 2005...we would have to go
  maybe for the Journal of Formalised Mathematics)
- use lower-case letters where appropriate in order
  to make Markus happy
- add tests for adding theorems to the various thm lists
- Check all the places where we do "handle _"
- We shouldn't use the command 'quotient' as this shadows Larry's quotient.
  Call it 'quotient_type'