private boolean isAlwaysNullBefore(Stmt s,Local base,LocalDefs defs){
  List<Unit> baseDefs=defs.getDefsOfAt(base,s);
  if (baseDefs.isEmpty())   return true;
  for (  Unit u : baseDefs) {
    if (!(u instanceof DefinitionStmt))     return false;
    DefinitionStmt defStmt=(DefinitionStmt)u;
    if (defStmt.getRightOp() != NullConstant.v())     return false;
  }
  return true;
}

private boolean isAlwaysNullBefore ( Stmt s , Local base , LocalDefs defs ) { List < Unit > baseDefs = defs . getDefsOfAt ( base , s ) ; if ( baseDefs . isEmpty ( ) ) return true ; for ( Unit u : baseDefs ) { if ( ! ( u instanceof DefinitionStmt ) ) return false ; DefinitionStmt defStmt = ( DefinitionStmt ) u ; if ( defStmt . getRightOp ( ) != NullConstant . v ( ) ) return false ; } return true ; }
checks whether the given local is guaranteed to be always null at the given statement
