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The algorithm is complicated by the introduction of control-flow jumps like ''goto'', ''break'', ''continue'', ''return'', and exception handling. Any statement that can be the target of one of these jumps must intersect its ''before'' set with the set of definitely assigned variables at the jump source. When these are introduced, the resulting data flow may have multiple fixed points, as in this example:
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Since the label L can be reached from two locations, the control-flow equation for goto dictates that ''before''(2) = ''after''(1) intersect ''before''(3). But ''before''(3) = ''before''(2), so ''before''(2) = ''after''(1) intersect ''before''(2). This has two fixed-points for ''before''(2), {i} and the empty set. However, it can be shown that because of the monotonic form of the data-flow equations, there is a unique maximal fixed point (fixed point of largest size) that provides the most possible information about the definitely assigned variables. Such a maximal (or maximum) fixed point may be computed by standard techniques; see [[data-flow analysis]].
An additional issue is that a control-flow jump may render certain control flows infeasible; for example, in this code fragment the variable ''i'' is definitely assigned before it is used:
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The data-flow equation for ''if'' says that ''after''(2) = after('''return''') intersect after(''i'' = ''j''). To make this work out correctly, we define ''after''(''e'') = ''vars''(''e'') for all control-flow jumps; this is vacuously valid in the same sense that the equation ''false''('''true''') = ''vars''(''e'') is valid, because it is not possible for control to reach a point immediately after a control-flow jump.
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