3.1.2. Drawing
a reaction step rule

A reaction step rule is built from expressions, charges, electrons,
lines and arrows. They are explained in 1.3 Reaction step rules.
Expressions and lines extend symbols and
bonds. They act like metacharacters and are interpreted
in the computation when the reaction step rule has been
added to
the list of rules. Arrows are needed
to express the movements of bonds, charges and electrons that appear
in the mechanisms of organic reactions.

To place an *expression* on a point
of the window area, first choose the appropriate operator from
the menu **Rules**, if one is needed, then
press the mouse button with the cursor on that point. A pop-up menu
appears, from which a symbol can be selected to complete the expression
(provided there are at least *51* pixels
from all other previously drawn points and
at least *20* from all previously drawn lines).

The operators that can be chosen from the menu **Rules**
to place an expression have the following meaning:

The asterisk ***** matches
any symbol. The expression

appears
on the window area.

**not** excludes exactly the symbol chosen in the pop-up menu.
The expression

is
displayed on the window area, if the symbol **X** is chosen.

**greater** includes exactly the chosen symbol
and all higher symbols. The expression

appears
on the window area, if the symbol **H** is chosen.
Here are the symbols ordered from lower to higher. Implicit sequences
like **Lr**,...,**Ac** mean the corresponding sequence in the
periodic table of the elements.

**Lr**, ..., **Ac**, **Hg**, ..., **Hf**,
**Lu**, ..., **La**, **Cd**, ..., **Y**,
**Zn**, ..., **Sc**,
**-**, **+**, **H**, **O**, **N**, **C**, **M**, **P**, **Q**,
**R**, ..., **Z**, **He**, ..., **Ra** (excluded
all elements listed before), **A**, **D**, **E**, **G**, **L**,
**R0**, ..., **R9**, **M0**, ..., **M9**.

**or ***s* means the symbol *s* or the symbol chosen
in the pop-up menu. The expression

is
displayed on the window area, if *s =* **X** and the symbol
**Y** is chosen. In this case the expression means **X** or **Y**.

**not or ***s* means neither the symbol *s*
nor the symbol chosen in the pop-up menu. The expression

is
written on the window area, if *s =* **X**
and the symbol **Y** is chosen.
In this case the expression excludes the symbols **X**
and **Y**.

**cancel** cancels the previous choice from the menu
**Rules**.

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