pretty names in substitutions in trees when printing
more clear test function
Extending of TreeInterpreter to work with if-expressions
Coding type checking of simply typed lambda calculus in SLL
Merge branch 'master' of github.com:ilya-klyuchnikov/sll-meta-haskell into ilya

* 'master' of github.com:ilya-klyuchnikov/sll-meta-haskell:
  Coding type checking of simply typed lambda calculus in SLL
  Extending of TreeInterpreter to work with if-expressions
  more clear test function
  pretty names in substitutions in trees when printing

Conflicts:
	Interpreter.hs
Added tests.hs