Kentaro Kikuchi Dual-Context Sequent Calculus and Strict Implication. 87-92 2002 48 Math. Log. Q. 1 http://dx.doi.org/10.1002/1521-3870(200201)48:1<87::AID-MALQ87>3.0.CO;2-N db/journals/mlq/mlq48.html#Kikuchi02