Action:

||give true
|and
||allocate a cell then bind "v" to it
thence
||check (it is true) then store 0 in the cell bound to "v"
|or
||check not (it is true) then store successor(0) in the cell bound to "v"

The first part of this action (before thence), gives the transient true, a bind from v to a memory cell and the allocated cell. The second part (after hence) works with the results of the first one, checks the truth-value of the transient and assign a number depend on that value.