====== SPO ======

<code java>
==>>
</code>

The Single Push Out (SPO) Graph replacement rule replace one sub-graph by another graph. The connections between the host graph and the newly-inserted graph need to be specified by the programmer. That means that if, e.g., in a one-dimensional graph ''[ A B C D ]'' the node ''B'' is replaced by the node ''G'', then the original connections (in- and outgoing edges) of ''B'' to the host graph will not be maintained. If an edge going from ''A'' to ''G'' is not explicitly stated, then there will be no connection between the nodes ''A'' and ''G'' in the rewritten graph. This entails that the nodes ''G'' (formerly ''B''), ''C'' and ''D'' will be no more visible, which means they are effectively deleted. Only the (unconnected) node A remains. ''%%Axiom ==> A B C D; B ==>> G;%%'' In order to achieve the same result as in the example above (Lindenmayer-form), it is necessary to explicitly list the connecting edges from ''A'' to ''G'' and from ''G'' to ''C'' on the right-hand side of the rule: ''%%Axiom ==> A B C D; a:A B c:C ==>> a G c;%%'' This rule type is frequently used to delete sub-graphs. ''%%Axiom ==> A A [ B A A A ] A A [ C A A ] A; B ==>> ;%%'' The resulting graph reads as follows: ''%%A A A A [ C A A ] A%%''.