wikipedia:en:Proof_system wikipedia:en:Sequent_calculus wikipedia:en:Axiomatic_system http://plato.stanford.edu/entries/proof-theory-development/

wikipedia:en:infinity wikipedia:en:Non-standard_analysis wikipedia:en:Ordinal_number

http://plato.stanford.edu/entries/epsilon-calculus/ wikipedia:en:Wh-movement#Extraction_islands wikipedia:en:Logarithm wikipedia:en:Tensor_product