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GroIMP includes four sorts of rules.

| Rule symbol | What it does |
| %%==>%% | Rule in Lindenmayer-form. Graph replacement rule, where one subgraph (often only one object) is replaced by another graph. The connections between the host graph and the newly-inserted graph are recovered. That means that if, e.g., in a one-dimensional graph [ A B C D ] the node B is replaced by a node G, then G will be inserted into the graph with the original connecting edges to the neighbourhood of the node B: [ A G C D ]. The blank space indicates here a successor edge in XL. Axiom %%==>%% A B C D; B %%==>%% G;|
| |**Note:** This is a different type of rule than the instantiation rules (which also use %%==>%%)|
| %%==>>%% | Rule in SPO-form. Graph replacement rule, where one subgraph is replaced 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 subgraphs. 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''. |
| %%::>%% | Update rule. This rule type does not change the structure of the graph. It is used to change the attributes of the objects (nodes) of the graph. c:C ::> {c[length] = c[length] * 20; } |
| %%==>%% | Instantiation rule. It is NOT used after a query, but after a class / module declaration. It adds geometric (or other sort of) information to all nodes of the declared type. |