There are some background papers by other authors. They used to be in
documentation/papers, but for copyright reasons they have been moved to our
private subversion: https://mathdox.org/svn/repos/private/formulaeditor/papers/

Titels and authors are below.

An Investigation on the Dynamics of Direct-Manipulation Editors for Mathematics 
  Luca Padovani and Riccardo Solmi
  in A.Asperti et al. (Eds.): MKM 2004, LNCS 3119, pp. 302-316 2004. Springer-Verlag Berlin Heidelberg 2004

XPress: A Novice Interface for the Real-Time Communication of Mathematical Expressions
  Marco Pollanen, Thomas Wisniewski, and Xiao Yu
  TODO reference

Squibs and Discussions, Memoization in Top-Down Parsing
  Marc Johnson
  Computational Logistics, Volume 21, Number 3, 1994. Association for Computational Logistics, pp 405-418

Natural Editing of Algebraic Expressions
  Jean-François Nicaud
  TODO reference

Linear syntax for communicating elementary mathematics
  C J Sangwin, P. Ramsden
  TODO reference