A generalization of boson normal ordering
  In this paper we define generalizations of boson normal ordering. These are based on the number of contractions whose vertices are next to each other in the linear representation of the boson operator function. Our main motivation is to shed further light onto the combinatorics arising from algebraic and Fock space properties of boson operators. 
Toufik Mansour, Matthias Schork, and Simone Severini