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A reaction
proceeds in steps
and each step has its own mechanism.
The mechanism of a step is represented by a reaction
step rule containing a pattern
to be matched by a corresponding image in a formula
and the instructions for the
movement of the corresponding bonds, charges or electrons.
In a pattern, which is a more general representation than the representation given by a formula, expressions are possibly connected by lines and are possibly carrying a number of positive or negative charges and of electrons. The instructions are given by arrows between lines, expressions, charges and electrons. Subsection 3.1.2 explains how to build a reaction step rule with the Graphic Composer. The reaction step rule instructions are applied to a formula whenever there is a matching correspondence between the expressions of the rule and the symbols in the formula. Every application of a rule preserves the symbols as well as their valence (the sum of the values of its bonds to another symbol increased by the number of charges and electrons). For example the reaction in Figure 1.2 can be seen, to proceed in two steps. First the ion adds to the carbonyl of the other molecule, then the resulting compound eliminates OH^{}. The corresponding reaction step rules are represented in the Figures 1.4, 1.5.
