refine list.stack
deprecate lambda expression of the form "x -> expr" (#54)
refine tests (#54)
resolve environment issues (#50)
now all closures in List object that involves non-standard evaluation evaluate expression in parent.frame()
remove all envir arguments in functions that involve non-standard evaluation (closes #53)
refine List using quote(data)
update readme
update vignettes
improve code structure
implement generic functions for List environment
update package info
update readme
update readme
update readme
update readme
update docs