This Moodle question behaviour was created by Tim Hunt of the Open University.
It is like the standard deferred feedback behaviour, but with the addition of a 'Check' button that the student can click to get Moodle to check the syntax of their response. This question behaviour was created for use with STACK https://github.com/maths/moodle-qtype_stack/
To install, either download the zip file,
unzip it, and place it in the directory moodle\question\behaviour\dfexplicitvaildate.
(You will need to rename the directory moodle-qbehaviour_dfexplicitvaildate -> dfexplicitvaildate.)
Alternatively, get the code using git by running the following command in the
top level folder of your Moodle install:
git clone git://github.com/maths/moodle-qbehaviour_dfexplicitvaildate.git question/behaviour/dfexplicitvaildate
For full install instructions, see the STACK install instructions.