http://www.realerthanreal.com/autolisp/
http://radsite.lbl.gov/radiance/pub/translators/dxf2rad.lisp
https://github.com/geetarista/autocad-routines
http://zwcad.com.tw/nick/lisp/lisp%B3t%AB%D7%B4%FA%B8%D5/Benches/Vl/
http://4d-technologies.com/techcenter/geom/ge_getnormal.lsp
http://4d-technologies.com/techcenter/geom/