A dual notion to available is busy.
A variable is busy or live if it will be used before being defined again; otherwise, it is dead.
A variable is busy on entrance to a block b if it is used in block b before being defined, or if it is not defined or killed in block b and is busy on exit from b .
A variable is busy on exit from a block b if it is busy on entry to any successor of b .
We can define a bit vector referenced, meaning that an expression is referenced in a block before being computed or killed, and solve equations for busy on entrance and busy on exit in a manner analogous to that for the available equations.