before mouseEnter ; put false into allowDrag; end mouseEnter

[erikhans08]

http://tactilemedia.com/blog/2015/12/06 ... line-demo/
in msg: go url "http://tactilemedia.com/blog/bezier"

A Scott Rossi "bezier_test" stack developed from work by
"Jim Hurley, master of mathematics, and graphics pro Alejandro Tejada"
has handlers like this:

local allowDrag
before mouseEnter
put false into allowDrag
end mouseEnter

The handlers are shown on the left in the CodeEditor.
On..., command..., and function... are what I have seen.
Is there any documentation or discussion on the before... handlers?
They look like they could come in handy with the "move" command.

Thanks,
Erik Hansen