** a desired property of some state does not yet hold (e.g., probe result)
** a desired property of some state does not yet hold (e.g., probe operation not yet finished, hence probe position still invalid)
Note added 8-8-2012
It just occurred to me that in the algorithm outline above, I implicitly assumed a strict separation of parse and execute phases for a block; during parse one would determine precondition and taint sets, sync if required, only then execute the block.
This assumed separation in the current rs274ngc code doesnt exist, in particular wrt expressions: expressions and assignments are evaluated at parse time. This implies that either syncing must happen on the fly during reference to stale parameters during expression evaluation, or expression handling is separated into a parse tree generation during read() and parse tree evaluation during execute().