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It would be very useful to allow users to draw reaction in the applet.
At minimum it could be done by allowing to draw an arrow and a + sign.
The current releae (2.4) has reactions. Right-click on the structure and choose "Reaction". Just drawing symbols is not possible, since in JCP things have chemical semantics. So you need to tell whats reactant and product and jcp will figure out layout. For the upcoming release, there might be a core applet without reactions first and later the one with reactions.