add the relation definition in XLib.v, for smallstep semantics
add the smallstep semantics for AE & BE
add the strong_progress theorem for AE/BE's smallstep semantics
add test cases for write buffers
add most of smallstep semantics, but there are flaws to resolve
plan to have a set tid containing all current tids; put testing functions in a separate module
under new idea, rewrite a/b/c smallstep semantics and prove their progress property