** Availability of Expressions**

The expression * &alpha &larr a b * is
* available at a point p*
if the value of the variable * &alpha * is the same as the value of
* a b * computed at the point * p*.

The expression * &alpha * is * available on
entry* to block * b * iff
* &alpha * is available on exit from all immediate predecessors of * b *.

* avail _{entry}(b) = &cap_{x &isin &Gamma^-1 b} avail_{exit}(x) *

The expression * &alpha * is * available on exit* from block * b * iff
* &alpha * is available at the last point of * b *.

* avail _{exit}(b) = (avail_{entry}(b) - kill(b)) &cup comp(b) *

In general, a system of simultaneous boolean
equations may have multiple
consistent solutions. It is necessary to compute the * maximal solution*
of the set of boolean equations for intervals at all levels of the derived
graph.