Concrete on a vertical wall is removed for resurfacing in the substantial absence of microfracturing by a crawler apparatus that is lowered from the top of the wall to the bottom by remote control. A lazy pre-image computation method is disclosed that builds new transition relation partitions on-demand only for relevant next internal variables of a state predicate. This user has the needed native privilege to add it to the system. The provider then executes the operation associated with the operation code and generates a return value. Patient-activated rate or dosage adjustments can also optionally be enabled.