The EXPOSED Set
A definition is exposed in a node if a statement in the node defines a variable and no subsequent definition of that variable occurs in the node
Thus EXPOSEDn[i] is true for node n if definition i occurs in node n and no subsequent definition for the variable defined in definition i also occurs in node n
PRESERVED and EXPOSED information can be easily constructed by examining the definition information for the statements in a block in their order of execution